He sounds as if he is upset ,instead of focusing on punishments how about you find out why he is behaving like this. Could it be he feels some jealousy to the attention your new partner 's child gets or are they treated equally ,does he get on with your new partner, is she kind to him, it must have been traumatic for him to loose his own Momat 4 year old, is there no possibility he could visit or see her from time to time.Focus on his positive side and praise him when you see him doing something right. Do you take him out ,you and him,, sports, games ,outside activities having Fun, If you hit a child you are teaching him it is okay for him to hit ,this could be part of the agression you mention..