Ah ching
I think at this point, you should not hit him back. I would try this: Put him against a wall or door, hold his hands down to his sides (so he can't hit you), look at him eye level (you squat / sit down on small chair), make sure he's looking at you (making eye contact), let him continue crying, wailing, whatever, when he stops to breathe, speak in a firm (not shouting), low, FIERCE voice and tell him "No hitting. No. Mummy said DON'T do that. It's WRONG. It's painful. It makes Mummy sad." Restrain him until he calms down (might take a while). Make him say sorry (can he talk yet?). Then when he's calmed down and said sorry, give him a hug.
can also make the room as dark as possible (but make sure both of you can still make eye contact) to set a serious atmosphere so that he knows you mean business.
we are still stronger than them, no matter what, so if he doesn't let you pin him down against a wall, try it on the bed - one arm hold down his hands, one arm hold down his legs, make the same eye contact and makes sure he knows you are punishing him. If he keeps getting away with it, it will go from bad to worse.
I really feel for you. Gotta stop this as early as possible.