Can't say I ever stretched when I was younger. It was something that I only thought about as I got older and less flexible and I wish I had started younger but at that age you think you'll be fine.
I can't tell him it would be good for him as no way he'll listen to "mum". If the coaches tell them this is what you have to do he won't question it. I think they are working hard enough and doing things repetitively enough now that he could do with balancing all that out with stretching, even if it is just starting good routines for when really necessary.