Straps allow the player maximum flexibility to find his or her preferred instrument position. A harness tends to dictate the position more, in exchange for broader weight distribution. I think that the best approach is (1) while using a neck strap, learn to play the sax in a position that is comfortable, ergonomically correct, and consistent with the teacher's recommendations; then (2) later, if desired for comfort, try to find a harness that will enable the player to reproduce the optimum playing position. Some of them may not.
I don't think I'd worry about the weight unless his alto is unusually heavy, and student horns do tend to be lighter than pro horns. He's not going to be playing 3-4 hours at a time, is he?