Used to do a bit of sprinting myself 200m. Sounds like a good sprinter at the age of 15. What you could do (what i did when i was a sprinter) is invest in a parachute, or running with resistance, keep running the distance required with the resistance until they can run it at about the same time as without it. Take the resistance away and in theory, they should run it faster. What is important though with any sprinting/running event is that you keep the core strong. If your son is kean to improve further, invest in the peak performance running and sprinting books from the book shop they’re brilliant for training programmes etc. Hope that helps.