from viral infection with a secondary/subsequent pneumonia
is quite common. The pneumonia can be viral or bacterial. bacterial needs to be treated with anti-biotics and normally viral resolves by itself.
Regarding playing hockey, from your notes he seems fine and it all depends on how he feels with regard to his well being. If he is back to his normal self, energetic and ready to go for a heavy exercise game like hockey, why not ?
I would recommend if he is feeling upto it, to play half the time of the game like 30 minutes with lots of oral fluid hydration before during and after the game. As I said, it all depends on how he is feeling at the moment. This is not a condition that requires prolong recuperation specially in young person.