Your son is 10. He hasn't learned to deal with certain emotions yet. Tell him to suck it up. The reason poly can hurt you has little to do with technique and more to do with shock load. Poly's stiffness transfers more impact energy to your body. Compounding the issue is the low power makes you feel the need to swing harder. Which means even more shock to those tendons/muscles. Just because we have the technology doesnt mean it's necessary. Especially a ten year old who could not possibly generate the racquet head speed necessary totaled full advantage of stiffer strings.
PS85, 367g/345sw, Pacific classic@58