he's gonna be under restrictions for a certain amount of time, so he's going to have to "rehab" to keep his strength up.
My experience is not with young athletes, but typically with the patients I have seen who have developed clots they have to be on anti-coagulants for 3 months to prevent another clot. I don't know if the protocol is different for a young, otherwise healthy person though.
... and is it dangerous to be playing football on blood thinners? It doesn't sound like a great idea but what do I know.