Knowing Bob Zemeckis and how detail oriented he is my guess would be that this was intentional subversion.

@AVARY Ive always seen it as him pointing at the dog Einstein. I mean, in the rest of the scene the kid keeps looking at and smiling at Einstein. The kids focus seems divided between Marty and the dog. Pointing at his dick seems like a weird stretch. But, whatever. lol

@BobDobbs656 I can totally buy this take! Nice!

@AVARY The kid does seem squirrely though. lol. Don't know if he was directed that way or chosen for those attributes (likely both) but he does stick out in the scene. He has a Huckleberry Finn vibe. Dances to the beat of his own drum. lol Adds a certain charm to the scene.