I think you need to guage how he might react to you telling him you are aspergic. What is the likely effect of disclosure on your relationship? A good way to tell if he will react positively is how open-minded he is and how he treats people who are different from the norm. Most of the time, I don't tell people I'm aspergic, because due to lack of knowledge about it, most people would take it negatively. If my awkwardness comes through, I just blame it on my "geekiness" which is viewed more as a neutral thing but still helps them understand that I'm not stupid, just socially awkward. If you're unsure, I'd say hold off on it until you know him better. If you feel you must tell him because you want to open up, be prepared just in case he doesn't know what to do with the information. I would probably tell him by explaining how I found out I was aspergic casually, as if I was telling him about my day. This way it's not so emotionally charged and more likely to be taken positively.

Hmmm, I didn't get my dx until recently (within the last year) and so have only had one relationship in that time, but I think you should tell him like you'd tell any other friend.

I don't know how you usually say things obviously, but I'll tell you the approach that has worked for me, and maybe that can inspire you to think of something.

When I enter a conversation with a friend, with the specfic intent to tell them about me being an Aspie, I usually say something like "You know, you may already have figured this out, 'cause I'm sure not the most ordinary girl around, but I have Asperger's, which is a kind of autism, so in case you were wondering about my strange little quirks - that's why. Just so you know."

And if there's a need I add: "So when I do x/avoid doing x, it's not about you and nothing personal. It's simply because (not) doing x would cause me to have a meltdown/stim for hours/be unable to get out of bed tomorrow/[fill in the rest as applicable]"

If I don't enter the convo with the intent to disclose, those are usually the convos where I bombard the poor interlocutor with more information than they can process, because my words and info-level has not been prepared for the occasion, so it's akin to opening the floodgates. It could easily go like this:

Friend: "Hey, speaking of your awesome costume at [other friend's costume party], what with your outrageous colour of nail polish I couldn't help but notice your nails. Have you always bitten your nails? I never noticed before."

Me: "Uhhh well, yeah, you see, I, uhhh, bite my nails when I'm stressing out. And I know they're not pretty, but having been told all my life that my bitten nails are ugly and wrong, painting them in outrageous colours is my own defiant way of saying 'fuck you' to all those who would judge me for it."

Friend: "Oh uhhh wow. That's a whole lot of thought about nails. I was just sayin..."

Me: "Yeah, I know. It's just that me biting my nails is not a bad habit I just need to cast off like so many people think. It's something called stimming. I do this because I have an ASD and therefore my stress levels go up way more often and way higher than most other people's. So I need to relieve this stress in some way, and for me it's biting my nails."

Friend: "Wait... what? You're autistic? But you seem so normal!"

Me: "Yes. I know. And you probably think that's a compliment, but it's really not, 'cause you have no idea how much energy I expend on appearing normal, just so people who are not friendly, like you, won't shun me and bully me and throw me out of their shops and such.... [continue long lecture on autism, misrepresentation, disability rights and subsquently my interlocutor's ears burning with all the traffic they're receiving]"

In the latter example, replace costume nail polish and bitten nails with any quirk, stimming, special interest, behavioural pattern, anything else, that an acquaintance might remark on.

I much prefer the former example, 'cause the latter tends to overwhelm people, but unfortunately the latter is quite often triggered these days, because people who have met my parents before meeting me, have been told of my diagnosis, and they very often jump right to the "But you don't appear autistic/you seem so normal!"-response triggering my lecture-mode.

Rather than attempting to curb this response - which I find very hard to do - I am now trying to maintain a persistently prepared, albeit brief 'lecture' that will tell them what they need to know and not bore them to death. If they really want to know - they'll ask.

Wow - that became longer than I intended. Sorry about that. I hope you can use some of it for inspiration. And I hope you do find a way to tell your bf.

You may not want him to treat you differently - romantically and friendship-wise I don't think he will if he's as open-minded as you say. But you really do want him to be aware that if you say "Leave me alone, I need to be alone!" you're not doing the classic romance novel fishing for attention, but are rather in desperate an immediate need of being alone, and he should respect that without getting an explanation, because you may not be able to explain at that moment.

I have had good experiences with boyfriends who did not respect my boundaries and need for being alone - of course I didn't have a dx to 'justify' my needs, which would clearly have been necessary in their eyes. And I have had a boyfriend who respected my boundaries and left me alone, but he was perpetually getting hurt by my tone and wording, because in bad situations I become blunt. The diagnosis cleared that up for him, and he knew that I was not being an inconsiderate ass when speaking to him like that, but rather I was in a bit of a crisis and everything was still cool between us even though I did not have the presence of mind to be nice to him(or anyone).