Your son will have to submit whatever is required of any ED applicant. He can probably skip any optional essays. I would recommend he still put his best foot forward in the app as the AO, not the coach, has the final say.
Your child is not “in” until he gets an official acceptance letter from the AO, completes his senior year without any academic or personal issues triggering a rescission. Interesting question on injury. I think there is more of a risk if he gets injured prior to receiving an official acceptance, assuming he otherwise would not have gotten in without the athletic hook. If he gets injured after getting accepted, I don’t think there is grounds for rescission.