What's helped me, more than just about anything with my sufferer (we both fit into that category) is to just focus on the friendship, rather than have "relationship" expectations.
I had to let go so many times. Firstly because he wasn't up for it. Was terrified and wary of being involved with a woman.
But was needing a friend.
The lack of expectations helps a lot, in fact, it's the only way it's possible, IMO.
We live together now. We are 10 years strong and going well now.
For us, our very traumatic pasts helps us understand each other. But I had to totally let go so many times.
If it's meant to be, it will happen.
Follow your own heart and listen to what he wants as well. Respect his need for space. That is one of the things that will help him relax the most.
Come from your heart when you talk to him.
Tend to your own needs. Don't put him in a position where he feels pressured or put upon to fulfill your needs.
Cry and grieve if you have to. Don't try to fill the void with him. Be prepared to take full responsibility to fill your own void.
Just be a good friend. And be a good friend to yourself.
If it's meant to be, it will happen effortlessly, as long as you can let go, stay in the moment, treat yourself with the most kindness FIRST.
You know what the cliche says about the road to love. It's rocky. It's not smooth. It can hurt. It takes courage, lots of it. It is risky business. It means coping with uncertainty. Lots of it.
Follow your heart and be kindest to yourself and don't blame him or make him responsible for how you feel.
He may disappoint you lots and you may have to develop a saintly amount of patience. If you think you've got what it takes, great, but, remember, you deserve your own consideration FIRST. Then, if you still have energy for reaching out to him, do it without expectation of an outcome, or you could be setting yourself up and you might resent him and that will surely be a recipe for heartache.
Many of us sufferers need lots of time to ourselves. We are often not very social people. So going out on dates a lot? Nope, not something many of us enjoy.
It will probably be hard work with lots of energy being put out and sketchy returns. But if he is your best friend? You will not put the relationship pressures on him and it might work out.
Good luck.
Opening your heart to someone is the bravest thing. Be prepared to feel everything. It will, most likely, be the most intense emotional rollercoaster of your life. So make sure you have self care practises in place that don't rely on him. You have to fill up your own cup. And that counts for whether it develops or not.
Also, one more thing, don't fall into the trap that you tell yourself or think you can make things better with your words. Sometimes, words make it worse. Just being there when he needs a friend is better. Maybe a hug or a hand squeeze, but, sometimes any words hurt and don't help.
You can't, necessarily, make it better or help any of his pain go away. Just being a solid, reliable, friend is probably what he needs more than anything, but, don't try to change him or "fix" him.
Does any of this make sense? Just take what rings true and helpful, and leave the rest.