You really love this guy.

If someone doesn't want help...there's not much you can do.

You have to get him to the point where he wants to be here. And that is a ton of responsibility to give yourself...I don't recommend (for you own mental health) that you do this....Honestly I don't. No. Just no.

The only thing you can do is to do what you're doing. Be there. Be quiet. Hold his hand. Give him a sense that you will be there for him. He will push you away in lots of ways. Some you won't even see that that's what he's doing....
You could try helping him feel good by "fixing" some problems that you make up...simple ones that won't overwhelm. Ask his advice on a few minor things. (very minor...you don't want to need, but you want to want). He will either help and be glad that he did, or he will close down.

I really don't know any way to make someone understand that they can count on you other than just being there no matter what.

But please, don't risk your own mental health for someone else. In other words, if you can't finish this (and not many can) then don't start.