He likes you but he doesn't care if he's in a long-term serious relationship or remains single. I'm kind of confused by this. Why did he say this? Did he say this because he doesn't see his relationship with you as going anywhere? Or is this just his feeling right now? Does he think that it could go somewhere, but he isn't sure yet? It just seems a little too vague, really.
For many of us, relationships can be really quite complicated and difficult, and we have a hard time really expressing what we want and/or need. This is definitely the case for me. I have only ever been in two serious relationships. I never understood how some people could meet and connect so easily. I never understood how some people could even want to do that. It's not that I never wanted to connect to others. But, it was that I found it just way too much effort than I cared to put in. Maintaining relationships, any kind of relationships, is really draining and causes me a lot of stress.
That being said, I am now happily married. I can't explain why it worked how it did, but it just did. *shrug* What I can say is that when I met my husband, I wasn't looking for anyone. In fact, I was of the mind that I didn't want to be with anyone again for a very long time (I was still married, but I fell out of love with my husband). But I met the right person when I wasn't expecting anyone even remotely interesting to waltz into my life.
Anyway, my point is, autistic people, even "mildly" autistic people, can have a lot of trouble with relationships. But a lot of other people can, too. I know neurotypical people who have trouble with relationships. Maintaining relationships is hard for anybody.
In your case, it's hard to tell if he even has trouble with relationships in general (it could just be because this is a forum and trying to talk about these things here is not easy!) or if he's just not ready for a serious relationship right now, or if he's not into you as you are into him. But clearly, this is kind of bothering you, so it is important you get some clear information from him. If it is really vital for you that you know right now if he sees this going somewhere and he can't answer that, then you have to move on. If he wants a little more time, then maybe it's worth giving him that, and revisit it again in the near future. But don't ignore your needs and what you want, just because you really like him and you want things to go somewhere with him.