Hard to say without seeing it. This can be a strength issue in the arm, in the shoulder, in the upper back, or a tech issue at heavier weights.
Without knowing more, I would start by recommending lots of push presses, especially from behind the neck, focusing on a very aggressive lockout position and 2-3 second hold at the top of every rep. Jerk supports might also be helpful. Jerks from behind the neck can help too to get heavier jerk weights from a position that should help him get locked out if it's more of a technical problem.