I’ve seen a child in preschool randomly hit himself during play time.
Infants cannot express their needs verbally. Therefore they using crying and sometimes even biting and hitting as a way of communication and getting your attention. Little ones will sometimes bite to show affection because they don’t know how to kiss. That said, it’s possible your son is hitting your back with his head as a way of getting your attention or even displaying affection.
It’s possible that this behavior is a form of self-punishment. Let’s say that a child has been lightly hit by a parent when he has done something wrong. The child might copy the “hitting” behavior if he feels that he’s made a mistake. Also…during play-time…kids will engage in “role-playing” where they basically act out various characters…and certain actions such as hitting/scolding/etc could be a part of dramatic play.
If this is happening frequently, consult your pediatrician. Meanwhile, perhaps these links will shed some light on the issue:
My child hits and bites himself. What should I do? | BabyCenter
Weird toddler behavior: Why do they hit themselves? - ParentDish
Circle of Moms - welcome to Circle of Moms
I’d say that either he is attention seeking or just showing affection, i’m pretty sure self punishment doesn’t come into it.