I say gentle and make his hand do gentle on whatever he hit/grabbed/yanked/pinched etc. If he bites me I say no bite, put him down & walk away. I do flick his hand if he is intentionally mean to our cat after I just showed him gentle. Like we just did gentle pets together, he looks right at me and then yanks her tail. And when he pulls my hair I pull his to show him it doesn't feel good.
Please stop flicking your kid and pulling his hair. That really isn't accomplishing what you think it is.