Put the situation in a position to avoid him opening up to you in the first place.
Fortunately for you, you have some pretty good information. You know he likes you before he's opened up to you. So there's a good way to do this in "the most gentle and least hurting way" without directly saying it to him.
Some helpful tips
NewLoveTimes has written an article named How To Friendzone A Guy Without Hurting Him. They offer 8 tips on how to subtly let a guy down gently. TheHealthSite offer pretty similar tips.
I've added strikethrough formatting to some of them because from the information you've provided you seem to be already doing them or can't do them currently.
The most commonly used form of friendzoning, this one is as obvious as it gets without having to make it ‘verbatim obvious!’ Dodge those romantic comments, flirtatious remarks or anything that inches even close to the lover’s lobby. Either use your humor to keep knockin’ off those attempts out the window or be plain ignorant. Whatever you do, never play along with the idea of you two being in any romantic relationship whatsoever, especially when you know the equation is using imaginary/hypothetical elements that will never be true!
Do not send out mixed signals
Stress on how much you value your friendship
If you aren’t (wo)man enough to tell him outright that you aren’t interested in him romantically, then just try and focus on how important he is to you as a friend and how you’d never do anything to jeopardize this friendship. Let him know that you cherish the bond you share and that you’d never want to lose it. Once he realizes that pushing matters up the lovers’ lane might result in cutting your friendly trip short, he might understand the risk associated with pressing the matter further and change his mind about pursuing you. However, if he can’t stand the thought of being ‘just friends,’ you should allow him his ache and let him be.
Refer to him with safe terms/terms of non-endearment
Limit the ‘talk-time'
Switch off the romantic
Talk about other guys
Offer to set him up
This is a tricky one. You’ve got to do it right without crushing his heart. Make sure you approach the process vis-à-vis gradation. Hang out in a group (which includes the girl you’d like to set him up with), give them a chance to hit it off, follow-up post the meeting, start the teasing, and gauge the responses. Then, pop the offer. You never know what might happen!
It's a difficult (and awkward for yourself, as you know and he doesn't know you know) situation for both parties involved, so the best way to avoid him "opening up" is to defuse it. (I wasn't too sure on even leaving Tip #8 on there. You're a sentient being, so I'll leave it down to you to choose whether that's a viable option or not.)
What If I do that and he still opens up?
Well, in the same article they have a pretty stern bottom line.
However, if he refuses to take the hints and the friendship flares too
quickly into a stronger and dangerous territory, it is a good idea to
sit them down and tell them, ‘I like you – as a friend. I’m sorry but
it would not be anything more!’ be truthful
Being direct and honest is the best approach.
But, there are also a few things to not only consider throughout the conversation but the aftermath too.
During the conversation
Obviously, when the time comes you can't choose where it'll take place. But if you feel the conversation will steer that way, make sure you're somewhere private away from friends, or anywhere he'd feel really embarrassed. This can lead to resentment.
Based on some of the things said to me during my teen years and to my friends. When you're having the conversation avoid saying things like (some of these are for future readers, too):
- "I don't want to be in a relationship" when you do.
- "Maybe someday in the future" - no need for any false hope.
- "You mean too much to me as a friend for us to date" - This is a big no no, he'll take that as a cheap tactic to use your friendship to get out of it (when the time came)
- "It's not you, it's me!" - This is an outdated classic and simply doesn't work.
Naturally, he'll be emotionally hurt. There's no light-hearted way. It'd be not only a metaphorical stab to the heart for him, if you've known him for "already many months" but it'll also be a stab to his ego/pride. Here's a couple of things to do after having the dreaded conversation to run your friendship a little smoother.
- Give him some space to heal (but don't completely avoid him). Strange I know, but things will be awkward and you'll have to make the first step into ensuring that everything is cool regardless of the conversation you've just had
- Try and avoid discussing it amongst your friends, literally the worst thing for a dude to hear is that it's the centre of conversation amongst the group of friends you're both apart of, if it's "introverted and shy" he might feel a disconnection to the group because of it.
Just remember, he'll get over it in time. Aslong as you help him through and stick to being friends with him, you're the one who has to steer this situation the way you want it to go in order for you to remain friends. Don't avoid that playful banter you have with the other guys than you do with him, but also don't put too much attention onto not doing it? crazy. I know. It should be fine. I got rejected a few times throughout school and I'm okay (i think?!) now.