Has anything new happened recently to upset him, does he go to Day care or are you a stay at home Mom, good idea to remove hard toys, has he enough stimulation in his life they can do this behavior if they are bored with nothing to do, has he plenty of games and fun , does he go out and mix with other Babies, distraction works well so if you think he is going to do it, get him involved in some fun, reading , music,