My 23 month old son often bites me and pulls my hair. He doesn't do it out of frustration but seems to think it is hilarious and a fun game. I tell him no and walk away from him, I then explain what he has done wrong and ask him to do something gentle to say sorry. He will then do it again 30 seconds later. It seems to happen more when he is tired. I'm starting to get upset about it because it spoils lovely things we are doing together and makes me wary of cuddle time with him because it bloody hurts. Any advice?