Very delicate question. As you are asking directions about "how to phrase" things (as it belongs here in IPS), let's keep on track and analyze what you said/could say in (near) future.
You said
I just reassured him that I only see him as a friend
while this is undeniably and appreciably honest of you, please notice that it is not reassuring. You told him the exacty opposite of what he wanted to hear. You did have right to say that, but you cannot expect it to end up as a reassurance in his ears.
Now, let's analyze his following behaviour.
First, he
then started making passive aggressive comments about me
and, later,
he completely ignored me
This is a very frequent kind of behaviour with which many people handle this kind of rejection. First, is the "aggressive" phase, where he is only (blindly) perceiving your (rightful) rejection as "you attacking him with a low-blow", so, in his mind, you "deserve" being "mistreated". Then he realizes it's not the case, you can't be blamed for rejecting someone you don't want, so all he remains with is the "passive" phase, were he just defends himself by severing (or, well, trying to severe) the connections with you. You are the source of his own suffering, therefore he cuts the connection with that source of suffering. And, yes, if you ask me, because you
told him several times that if our friendship was bothering him because of the underlying feelings he has for me I could give him space, or stop talking to him for a while until he gets over me.
he could have been more "honest" and actually ask you for space (either definite or indefinite). But well, it was too hard for him, I suppose, and decided he follow the "not-really-honest" path, and we are called here to provide phrasing, so here we go.
As you do look very clear and straight-to-the-point (I personally do appreciate it) while also being very forthcoming (you do care about him) the best you could do, imho, is taking the decision he should have taken and state it with a simple, clear and direct
Look, this is bad for our friendship. You may agree or disagree, but I do see that when it comes to me you are suffering. So I'll do what's best for our friendship and - above all - for you and withdraw, I'm giving you some space.
Imo this is better said by voice or in person. Use a tone to make it clear it's a statement, a decision you took, not a request for opinion. With this statement, you
- prove you care for him
- go straight to the point (as I think it's very suitable and fitting for the way you've always handled it 'till now),
- take a decision he's apparently unable to take,
- go along with his "passive" phase of severing connections from the suffering source and, consequently, likely healing.
After that, stick to it. Give him time. If this is something that can be healed by time itself, he will come back.
I am telling this on personal experience. A girl I liked told me straight and went into oblivion: that quick communication and that space did help me a lot.