If I understand what you mean; its tipping nose up when he puts weight or pressure at the back. This can be caused by the clamp too far forward, maybe you can reposition it to the back side of the seatpost?, The big problem is that after it does this a number of times, and it gets jerked back up without loosening the clamp, the teeth on the clamp pieces get rounded off and they just wont bite together anymore no matter how tight you crank that seatbolt.
Take the clamp pieces off and look them over, Ill bet you see they are rounded, then got down to the LBS and buy a new clampset and life will be good.