Isn't this what some people call "the honeymoon effect"? Like Nick said, I've seen blogs and articles claiming that this is particularly common among Aspies, but I think it's actually common among NTs just as much. The fact of the matter is that you can't expect the pace of the beginning of a relationship to always last forever. It doesn't necessarily mean that his feelings have changed (although that's possible too), but you might have to readjust your expectations. Ask yourself whether you need that level of intensity? I can definitely see how an Aspie might front-load a relationship to the point of obsessiveness, but it's hard to maintain.
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After years of self-imposed exile. I am now making an effort to talk to people. So anyone feel free to PM me on any subject, I would love to try to interact with people more!