Tell him to clean his clippers befiore cutting your son too! The cut shouldn't matter as much as the procedures to prevent them. The individual strands of your son's hair are probably thick and has trouble clearing the opening of the follicle as it grows out. Add that to adding hair grease or pomade and the follicle gets clogged. As the hair tries to clear the follicle it will grow in on itself and get infected. It's basically and ingrown hair. Disinfecting the area, and the item used to cut the hair as well as using an exfoliating product daily after the hair is cut (to help unclog the follicles) will certainly get the problem under control.
I know plenty of men who would otherwise get razor bumps and if the barber CLEANS HIS CLIPPERS with alcohol before he cuts, and you apply tend skin once to twice daily after the cut - they don't appear. It also helps if he cuts in ONE direction in the back. Razor Bumps are just like bikini line bumps. You know how we shave (if we choose that method of hair removal) in one direction....? He should be doing that too!