"Yes, he's been resisting my attempts to draw him out, but he is definitely responding defensively. I've heard so much of his history and I can clearly see how a lifetime of building safety barriers has led to this behaviour."
What are you trying to "draw out" in your numerous "attempts"? What does he have to defend?
He doesn't owe you sex. There's nothing wrong with him not being attracted to you.
@ColdCatch is right - it seems you only agreed to a friendship so you could embed yourself in his life and collect information and change his mind and gradually become a couple.
Surely the lack of sex is because you're not a couple or dating or romantically involved.
You're not and have never been in an asexual relationship. You're a platonic friend who is offering yourself on a plate and he still doesn't want you.
I get the impression its been you suggesting a lot of the "date style" activity (trips, bed sharing, contact) and he's agreeing and going along with the flow.
If you stopped contacting him and planning things and turning up at his physically, he wouldn't chase you and you wouldn't spend so much physical time together.
He's a lot more important to you than you are to him.
If you decided to sleep in the spare room he wouldn't follow you in or invite you to his bed.
If you dated other men he wouldn't care or even be interested unless you instigated a conversation.
I've known people like this IRL and its completely obvious the friend they're fixated on isn't that into them in that way eventually ends up a bit scared of them. Their narrative is just all skewed.
They will suggest John goes on say a spa break (as friends, paying half) then afterwards say "John took me out on a romantic spa break, that proves I mean something to him".
John may have enjoyed the time and been polite and attentive and done the driving, but they certainly would be shocked at the other persons interpretation!