so, my son's uncle is 3.5 years old.
and every single time they play together, he hits my son on the head with a toy or something...
His parents give him time-outs or wtv. but it doesn't work, he literately will go hit my son again 1 min later
So he smacked my son today, and I smacked him.
You don't mention your son's age, but have you considered teaching him that when someone hits you, it's OK to hit back?
Some of us like to call that "self defense."