Time out worked when my son was younger. Not so much now that we are the same size. My son is nonverbal. I find physical activity like walking or swinging or swimming help him. There is a lot of guess work too since he doesn't talk. Physical aggression can be a sign of physical discomfort he cannot tell me about like a tooth, stomach, or ear ache he cannot tell me about. If I can guess the right one and address it, the aggression will lessen.

My nephew is 8 and he's started doing this the last 6 months or so. My mom has bruises all over. Time out doesn't work. Talking to him doesn't work. We don't know what to do either.

How old is he.
Have you tried like time out, something to show him there is a penalty for doing it. Tap the hand, time out, take gadgets etc