I'm not so sure thats the best choice. From the sounds of it, hes just getting into skiing, or hasnt been skiing for long. I think thats a shorter ski will help him get a feel for the park, and to find his own style.
Going too long at first can be a bit of a problem. Once you are more advanced, then a longer ski will help with your stabibility, landings, spins, etc.
I would say a 169 would be ok for this season, but if you progress a lot, you may find them too short the next season afert. Its a tough choice, but I wouldnt exceed 175 cm