All actions are a form of communication. Maybe a sensory issue? He might be trying to regulate something that’s an issue for him. When my son (same age and has autism) is self harming, I take him outside to play or give him a warm bath. Often he just asks for those strategies instead (in his own way, he’s nonverbal).

I'm not sure if this helps, but... Did it start when something changed? New environment (like preschool)? When is it happening-- always or at a specific time? After an overstimulating environment? All good questions to ask yourself if you haven't already.