Sometimes chucking can be as a result of the valve at the top of the stomach not being strong. In some babies it takes longer to strengthen than others. As long as he is otherwise happy and healthy and developing (and since he is seeing a paed and being checked for weight gain you know that's okay), etc, it isn't anything to worry about. The amount coming back up usually looks more impressive than it is.

His valve will strengthen in time and this won't be an issue. My DD constantly possetted (which is small amounts, but also genetic, since I was a projectile chucker as a baby!) until about 7 1/2 mths, when she suddenly stopped.