You can either not react as sometimes these things are meant to get a reaction out of you or you can definitely make a big deal and act super hurt so they know that it’s something serious and harmful
I don’t know if you are in the uk but uk the end I ended up having to get early help team intervention in the end with my son cause nothing was working maybe you could as for some advice
This doesn’t always work but sometimes pretending to cry a little might indicate to him that you’re “hurt”. Really try and focus on composing yourself (trust me I know it can be hard when you’re getting abused 😭) and showing him what gentle hands are by example. Take his hand and have him gently “pet” whatever he hit. We did thank still do this with our dogs but it seems to work when I do it to me. Repetition, repetition, repetition. It’s not something that can be fixed over night. My little girl has gotten so much better about it. She’s now almost 20 months.