Welcome, AR--very thoughtgul post!
Others here have speculated, from time to time, that Fredo's childhood illness may have affected him as an adult, but I don't think anyone has linked him to autism.
In the novel, Fredo (who's a much more minor character than in the films) is described as "inclined to dourness." But he's a major force in II (whose modern plot, as you probably know, didn't appear in the novel--it was created for the film). I've always felt that Fredo was drawn far too broadly--he's almost comically, satirically stupid. I doubt that FFC and Puzo intended any subtlety in Fredo, and giving him autism would have been far too subtle an explanation for his behavior. Then I came to realize that the broad stupidity made his treachery all the more treacherous--who'd have expected it from that dunce?

You know far more about autism than I do. But, good friends have an autistic son. When he was a little kid, one of his major symptoms was that he was hyperactive. As a teenager, the manifestation that I see is that it's hard for him to listen during conversations--he can't restrain himself from breaking in. I realize that those aren't the only symptoms of autism, but I didn't see either of those in Fredo.