Switching to a full choke will only help if your son was shooting beyond the effective range of the mod pattern, and based on your description of events I don't think that was the case. If he was putting the birds squarely in the pattern, even a weak pattern would kill at least 50% of birds hit. Four birds crippled leads me to suspect he was simply shooting a little behind and hitting the birds in the tail (alternately, he could have been shooting beyond the effective range of the shot, but steel 2's should be good to around 50 yards or so - even that square Blindside shot should be good to at least 40 yards
How far were most of the shots taken? That will give us a clue. Also, I would take the gun out and fire a couple of test patterns at the distance you most often shoot at ducks. After shooting, draw a 30" circle around the densest part of the pattern and post up a photograph. We can then give you a good idea of where to go next. It may be that he needs an more open choke as duckslayer74 suggested, but the only way to know for sure is to shoot some paper. Good luck!