My son is refusing to have surgery and my ex is backing him,why?

My 16 year old son has a partially torn ACL and his doctor is recommending surgery. Well my son is not pleased about it needless to say and has bluntly told me he refuses to have the surgery but yet he still wants to play football (he is a starter on his High school Football team). Which his coach won't allow till he is recovered and his doctor doesn't think he will fully ever recover but the surgery would give him a chance to play again. Well i asked my ex his dad to talk to him about it and he refuses saying its his choice and he can do what he wants. Well in all honesty i think my ex is just trying to cause problems with me and i don't understand why we divorced mutually. How can i convince my son to have the surgery? Advice?