When he is sober and in a good mood you might want to confront him one time to be sure you can't convince him of the help it provides. Show him some of the known facts, show him what states have legalized medical mj for purposes just like you have. It can't hurt...if not then I say this..

I say since you can't work for awhile after surgery you should lay off the weed for those two months to please your father. You might even try to make a deal with him that if you give up the weed, he gives up the beer. After that you should look into moving out on your own because if you are in pain and it's the only thing that helps or even if it's just something you enjoy, it's not his place to tell you at your age how to live your life. Trust me being out on your own is very liberating having your own place with your own rules not having to live by someone elses ways.