my son is ADHD-and he sounds just like yours.

he gets fixated on many things, too-one at a time, and i just let him go. he gets fixated on subjects, tho-last summer, for example-he watched the weather channel constantly, talked about it when he wasn't watching it-he knew more about meterology than most adults-but couldn't talk to someone his own age about ANYTHING.

thats one of the reasons i sent him back to public school-so that he could make friends and learn to be social.......some days it works and some days it doesn't.
i don't have any advice-i just understand.