Are you holding him when he does it? Or are you down on the ground with him? If you're holding him, I'd put him down with a firm no & an explanation, then he must say sorry mummy/daddy. If you're on the floor with him, stand up away from his level & do the same, firm no & explanation. This worked for us when Zander went through a stage of grabbing my glasses, so not the same but similar. Then next time you ask him to do something & he does it without hitting, lots & lots of praise!