LOL... my kids are horror movie freaks (Halloween is our favorite holiday - do you know how hard it is to find zombie costumes for babies??)... Harry Potter would look like CareBears to them! Stephen King shot his first 8mm film at the age of 7, I believe... if one of my kids will grow up to be like Stephen, I'll be happy... old mama taken care of in her old age! LOL!
As for the OP, I agree with many of the pps. Our family, our rules. I know my kids would be bored to tears with the Harry Potter movie. They haven't even seen the previous ones, so they'd have no clue about what's going on. Maybe you could consider letting him ride in the street while you or DP supervise him closely.