My daughter did that too!!! She was between 18mo and 2yrs I think. She did it to the point of vomiting twice, but after she would vomit she would stop. She only did it for a month or so and then stopped on her own. I think they just discover a new feeling that they can control, so they do it. He also might like the reaction that he gets out of you. When my daughter would do it, I would grab her hands and hold them down and tell her no. Usually she would do it in the highchair. Try to be very persistent and very serious when you tell him no, no matter how angry he gets. Don't go into explainations with him or anything just a stern no and a stern face. If you can't get him to stop ask you're doctor, but I think it's just a phase thing. Good Luck!!!