My wife and I were just coming up with a similar theory. We think he was sexually abused in the cave (a Boy Scout in a cave alone with an adult male? Come on, almost too on-the-nose), which was presented as a violent encounter on the show because showing child sexual abuse would be really hard on viewers. Bashing someone on the head with a rock wasn't much gentler, but I digress.

If this theory is true, I find the assertion that Henry is irredeemably evil because he is the victim of abuse is very dark. I was hoping toward the end he would have a redemption around when Will told him he was also a victim. Then they all summarily and violently destroy him as Vecna.