It sounds like you're trying to convince yourself that he does love you really he just has an ism.
It's impossible to tell from a description on the net, but the details that you've listed here don't immediately indicate ASD spectrum.
As you know ASD spectrum implies problems not only with communication and social interaction, but also awareness of other people's feelings, issues with empathy, restricted interests, repetitive behaviour and thought patterns, fixation on routine, problems with touch due to over-sensitivity to sensory stimulae.
Yet you say he's very affectionate, cuddling, stroking you, maintaining a lot of physical contact, buys you presents, makes you feel 'cherished' etc, is a great friend - putting himself out for friends, colleagues, family - all of which shows an awareness and consideration of other people's feelings, no problem with empathy, and no problem with touch in general and sexual contact in particular.
ASD types tend to feel overwhelmed by sensory information rather than the 'underwhelmed' as you describe his reactions.
If ASD types tend to have difficulty verbalising feelings, he's actually been very clear how he feels about you: affection and fondness but not love.
I'm not saying he's not ASD, I'm not qualified to make that judgement, and if I were, I certainly wouldn't attempt it over the net with regard to a person I've never met.
When you describe him as being from a buttoned up family and as being emotionally constipated, that does all make sense and at least fits with all the details. From what you've written here it sounds more like an emotional disorder than a neurodevelopmental one.
But as I say, I'm not qualified to make that judgement. In your situation I think I would try and accept how he says he feels about you, and ask him what that means in the long term for you. How does he see your relationship? Does he see it lasting etc?