I know how it feels as I am in pretty much the same situation.
Except that for the time being we are only seeing each other as mates, as the other day he told me that for the past few weeks he hasn't had all the feeling he had for me before
. But we will still see each other, like I said only as mates, and he hopes that his feelings will return to normal...I hope so too, he's my first boyfriend and we've only been together a few months.
So, we met up yesterday and at one point when we were in Pizza Hut, he said how quiet we had both been that day (and it was probably because it felt awkward and different to usual, etc.), and then he said "That's another thing; we never really talked much." And by that he meant we never talked to each other much before, and when I spoke to him on the phone a minute ago he said it was adding to the problem and how he felt, but at the same time if it wasn't like that he would still feel how he feels.
BUT - when we spoke I sort of had the intention of telling him about having AS, especially after what he said. I didn't in the end, however I said that I can find it difficult to start and maintain conversations, it's just part of who I am and that it does eventually get better, sometimes it takes ages, sometimes it doesn't. He said that he can be like it - well I couldn't really see how, he seems really chatty - but then again, only about theme parks - which he is obsessed with like I am - and I have AS...so I couldn't help thinking if he has it! Cos he said he can start conversations unlike me (and said it in a nice way of course!) but after a few sentences has no idea what to say next!!
So basically I feel loads better already and I haven't even told him, but the fact that I started to explain it has cheered me up so much it is mad! So I'd say gradually tell him...and you say he is shy too, so I'm sure that he will understand
I plan to tell him next time I see him, as I feel it would feel easiest for me. Another thing I would suggest is not texting him, personally for me it wouldn't feel right and not a very great way to explain it. Talking to people can be very difficult, especially when explaining AS to them, but for me, the thought of physically being with him when explaining makes it feel more...personal and closer, so I'd feel more comfortable. Whereas on the phone wouldn't be so, and e-mailing...well I'd want to be with him when explaining it. But that's obviously just how I feel, it's up to you what you wish to do, just giving you my thoughts, etc!
Good luck...I know I will need it, too! 