When I was swimming, the boys wore their regular trunks for practice and the team "uniform" suits which were more like bike shorts for meets (and us girls wore a second, looser suit - usually an old one where the elastic had started to give out - over our main suit for practice). The extra little bit of resistance from a less streamlined suit helps to build strength and endurance, though not every coach buys into that thinking any more.
I'd suggest you wait and see what your son's team expects before spending any money on suits. They aren't cheap, so why jump the gun and start buying things you might not even need?
ETA: The teams I was on and the local teams in my area now generally have preferred suppliers and give the team members coupons/codes to use with those suppliers at the start of the season, so that's another reason to wait and see. I always ordered my suits through the same company that supplied our uniform suit, because with the 20% discount for being part of a member team they were cheaper on plain old Speedo suits than any of the local sporting goods places.