Disclaimer: I use stronger words and controversial simplifications not to offend anyone but to emphasize nuances in details that softer words may cover under one word. I kindly ask careful reader to downscale the strength of the words while maintaing the difference.
Background: I am from Czech Republic and I live here. The society here seems a bit behind so called West Europe, for both better and worse. As a consequence men are expected to be "the strong ones", "the responsible ones", "the reliable ones" while women as "the caring ones", "the sensitive ones", "the emotional ones".
For 40 years all the people who differed from the normal society, including those with opposite political opinions, were imprisoned in dedicated assylums hidden in dense parks. One could'n meet a man on a wheelchair on a street; depression was a taboo etc. Handicapped ones were treated like invalid ones. This shit, and many more others, does not vanish overnight, even 30 years is too short to heal this.
I do have my demons, I know of (some of) them and I neither can fight them off myself neither can ask for help fighting them off. I have problems accepting help as well. I'm trying to identify with him and suggest what would help me, but I cannot ask for it.
Your SO is a Man, right? He must be strong and brave and invulnereable. If he would admit he is not brave, not strong, or vulnereable, he would fail. He would be a lesser man, nothing. Useless piece of crap that is not good enough to even walk in the sunlight. Trauma is supposed to slip off him as a water slips off a duck. Arm torn away is supposed to be a just a bloody nuissance...
This mindset what he thinks he is supposed to be and what he thinks he actually is, this clash of expectation and reality is tearing him appart.
Anytime anyone suggests he needs a help, which is same as suggesting a help might be beneficial, they undermine his masculinity and question his right to live, they increase the stress he is facing in his mind. As a result, it does not help him at all, he block himself in his comfortable zone because expelling this out his head reduces the pain to level he is used to.
On the other hand, in your eyes, he is a human being not a John Stonewall. But he cannot see through your eyes. Even imperfect (they who are, be my guest and thow a stone!) he is above any other for you, otherwise he won't be your fiancé. I humbly think he is not significant to you despite these imperfections; he is significant to you with these imperfections. His state of mind is not imperfection for you, but for him it is, but a sort of injury. Actually it is simillar to someone being hit by a javelin in their chest; it is serious, everyone see the javelin must go out but whoever touches it, it hurts like hell.
Time is both your friend and enemy.
The trauma happened 10 years ago, you know each other for a few years and you've found the trauma out months ago. You need to build trust in him; a lot of trust. He must trust you very much to tell you about it, he needs to trust you more to talk to you about it. He needs to trust you even more to accept your help. Only then you can actually help, otherwise your help may cause more harm. Do not push hard. Not doing anything will do harm for sure.
The alegorical javelin stuck to him will kill him. The javelin must go out, but immediate removal will kill him sooner.
Trust him. Share your trauma unrelated feelings with him. Share you concerns with him. Share your memories. Ask him for help. If you have your 13th chamber, let him slowly go in. Do this all casually, naturally. Ensure he is comfortable with it; start with easy ones and slowly go to things more and more sensitive to you. Let him know he has access to areas noone else have or ever had. Do not force him to make promises, let him know you trust him you don't need any promises at all.
He must trust they who try to remove the javelin do not want to and won't kill him.
Learn how to read him
Be empathetic. Learn how much he is comfortable or uncomfortable when asked or told something. Be able to find out how much you are pushing him out of his comfortable zone. You need to know when you are pushing, when you are pushing much and when you are about to push too much. Try not to push him that he will (ask to) end it. If you push him increasingly it is very beneficial to unload the stress smoothly as well. If he terminates it, he will terminate much faster.
It is needed, among other things, to know kow to distinguish small blead from serious ones and how to stop them promptly and effectively.
Build his self-esteem
He must see that he is worth better life than he is expiriencing so far. He must see that when he admits some wakness - failure in his eyes - at worst nothing changes in your attitude towards him. He must feel that he is of higher value for you as a human with feelings than a Joe Stonewall. Assure him noone cares he made a mistake. Assure him a mistake is not a failure, it is a lesson. He may think of himself as expendable; convince him he is scarce. He must se himself strong and worthwile and comfortabel enough to overcome the discomfort he is about to face.
He must know the pain will raise a lot but then disappear after the javelin is out.
Divide and conquer
Slowly undermine his blocks by asking about his concerns, his feelings, his memories. Start with trauma unrelated topics. Be respectful when he doesn't want to talk deeper. Be appropriately supportive when he starts on his own. Don't push hard, don't be oversupportive. Be sure, that you are few levels ahead, that you disclose more serious things than you ask for, that you trust him more than you demand from him.
Cut the javelin in parts and remove one after other. Leave the most critical parts for the end so you can remove them fast and stop bleading immediately.
Be strong, solid and respectful
Do not give up. Don't push hard, push little but steadily. Do not fade; do not give up the ground you already conquered. Whenever he stops telling you, stop asking. Whenever he seems uncomfortable, stop telling. Change the topic to something casual, like grocery shopping, work, any other usual small talk. Wait for a while. Let him process both what has been told and the emotions it caused. Let him relax. Then try to talk about it from other point of view, in other words or talk about something else, but keep the intimity same or little higher.
When a part is removed from the body there is no reason to return it back, isnt it?
Don't make it a mission
Yes, it is a mission, the really long one and exhausting one. Be sure you leave your mission accomplished celebration deep in your head. There mustn't be any fireworks, fanfares at all. Even if was your Biggest Succes Of All Times(tm), you must take it casually. He must see you did it all because of him, because youcare for him. Not because you wanted to prove something to yourself. Do not ask him whether it (all) was worth it - he must be the one who say it. Spontaneously. Do not expect rewards, do not expect anything. Accept any result.
Be sure you never celebrate major improvement in your quest. He mustn't feel like he is played with. If you say something like "(See,) was it that hard to tell me that/go to the doc?" he will lock himself and you'll waste all your effort and lose any other chance to help him. It is highly probable you will lose him and block a chance anyone can help him. Except for Death himself.
After succesful step, doctors don't celebrate, they just continue with another step; after succesful operation doctors usualy don't celebrate, they just go prepare for another patient.
I know the javelin analogy is not perfect, but I liked the idea and realized it's not that good when I wrote too much text. It may seem as Mission Impossibe, but hey, they made six films of that franchise!
I wish all the best and that I might help you a bit. Good luck.