I don't know your boyfriend, so I'm going to put the two different responses I can think of depending on tone. I have two different tones I could be using in saying that sort of thing and I'm not sure of his.
The logic tone. Sometimes people tell me I'm being cold for speaking logically, but they also acknowledge that what I'm saying isn't untrue. In that tone I could be saying "our end goals aren't really compatible," and mean that whatever emotions may be there, practically speaking it just wasn't likely to last. If that was his tone, I would recommend being similarly logical. Tell him you thought about what he said about it not being right, and explain that while you would want to stay and try if there were a chance it would work, you don't want to put time and effort into something if he's already decided it's not right. If he was speaking logically, I think it would be fair to do the same, and honestly that may be easier for him to hear than something more emotional.
If he was speaking from a place of anxiety, talk to him about it. Find out what he's worried about (is it a low self esteem thing? Why doesn't he picture a future?) and then decide whether it's a problem for you. If that's the case be gentler, but tbh it shouldn't be harder with an aspie than it would be with someone else. The only adjustment I'd make is maybe easing out of any patterns you have; if you usually talk for hours every day, and you feel you could handle it, it may help him if you still talk a little each day for a while to ease out of the routine; maybe text instead of skype, or 10 minutes instead of an hour, etc.
Personally, before I knew I was on the spectrum, I was obnoxiously rational about relationships (in some ways I still am). I would ask someone where they wanted to live and how many kids hey wanted before I'd be willing to go on a date (this is in high school, mind you) because I didn't want to invest emotional attachment if things weren't compatible in the big basics. I didn't date until I was 17 because I didn't understand wasting energy on something highly probable to fail. If your boyfriend is going by the probability of things lasting, it's not a reflection on you or the relationship, and it may be worth waiting a while longer and trying to understand those calculations he's making rather than seeing them as a judgment of the relationship. But if he doesn't want the same things you do and was articulating those differences, he'll be able to understand not wanting to stay for something that's not going to last.
All in all step one is to try to understand him. I'm hoping that you've done that and just didn't articulate it well here. Step two is to communicate. If you're worried he won't understand, spell it out. I always feel best if people explain things that they think are obvious; what they're feeling, what their reasons are, etc. Even if it didn't go over my head, it's a relief to have it confirmed and sometimes their explanations help me connect dots that I could see independently but hadn't connected. Step three is to ask what he needs - a gradual routine change, a maintained friendship, or total space. Step four and five (for which order depends on how much thought they take) are to tell him what YOU need from the breakup, and to figure out which of each other's needs you can help each other through. You don't have to attend to his breakup needs if they're too taxing for you, and don't assume he can give you everything you need. But talk it out clearly and directly, express how his needs affect yours or what's hard, and be willing to explain. He's a complete human being on his own, being an aspie doesn't change that; he shouldn't be devastated without you, and while it's nice that you're trying to be mindful of what being an aspie means for this, it's also a little concerning that you're asking a group of strangers rather than just being direct and honest with him.
