Guys, why would he do this?

I’d appreciate male opinion, but if a girl has a similar experience please share. I’ve been on and off with this guy since December, and when we are together he goes completely out of his way to assert his feelings for me, is extremely attentive and all around just a stand up guy. He says deep sh*t, like being in love and wanting a future, etc. But then when we are apart, it’s as if I don’t exist. He puts in no effort to see me, and blames it on him “allegedly” being autistic, so because of that he has trouble getting close to people. And yet he gives all that attention to his best friend (who is taken by their mutual best friend). Why would he bother doing this? Am I missing something? Am I insensitive because he’s autistic?
Guys, why would he do this?
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