While you are sitting on the unit, engine not running, have your son watch the belt action while you operate the clutch petal. When you push the clutch petal down, the belt should be extremely loose. As you let up on the pedal the belt should slowly start to tighten up. If all looks correct, jack or block up the rear end, put the unit in neutral and try turning the rear wheels together in the same direction. The wheel should turn easily. If they do, shift to first gear and try again. If both wheels turn together, repeat in each gear. If the wheels turn in all gears, I suspect the tranny is OK, but either you have a belt routing problem, or the brake is not stopping the tranny gears, to allow shifting.