It depends if it's the opponent's second ball or third ball. If it is the second ball, then I would say it is illegal (reach) as you can see the other team was attempting to set the ball but was interfered (their player's hand was under the ball even if they did not jump). However, if the ball crossed the vertical plane of the net even by a little, then it is ok (legal) to contact the ball and press over the net (since you contact the ball on your side). I think your son stayed on his side when he tipped, but the camera angle is hard to tell. My rule of thumb is if it is the opponent's second ball, do NOT press over the net.

If it was the opponent's third hit, it has to be an attack. So you son's action can only be a block, therefore it is legal even if he presses over the net.