I'm a 14 year old girl and I started Accutane when I was 13. Like you, my mum was worried too. My mum and I always used to fight whenever she brought up my acne too:( Reading your post, there is only one issue. Your son seems very active, and Accutane can give you such severe back and muscle pain:( I find it painful just to lie flat on my back or raise my arms above my head as it strains my back. So unfortunately your son may have to give up athletics for a while or just not go through with the treatment :/
I am on a 60mg dose, so 3 tablets a day, but if I feel I am in pain or my skin is way to dry, I either skip a day or only take 2 tablets.
Good Luck to your son, I know it can appear scary! As I am the same age as your son, I hope it gave you some advice?<3