First be sure there is not a physical reason that he forces himself to throw up.
Two approaches that MIGHT work.
1) totally ignore that fact that he is throwing up. Clean up the mess and do not speak to him about it. Don't rewared the behavior by reacting. Act like nothing happened. I had a friend whos child was litterally FORCED to eat and he refused anyway he could...when he came to my house he ate like a little pig because I told him he could eat or not...I didn't care. The refusal was just not given any attention.
2) Try to find something to get him involved in immediately after eating, a trip in the car, a game, something he would enjoy. And tell him that you are going to do this "special thing" immediately after we eat. Hopefully he will not have the time to prepare the upchuck.
And get yourself a diary to put somewhere it is easily written in and record what he eats when he does throw up, and any circumstances involved in that occasion. Maybe there is a pattern that you are not seeing.
lol