Without knowing specifics it sounds as though you need to put your foot down. A 15 year old may have some say in where they live in court but with the attitude changes, school issues and the drinking, I think a court would look very closely at his father. If he has up until recently been in your primary care then you legally have primary physical custody. Letting him “visit” his father for this extended period of time is alright but you should let him know that his father doesn’t have custody and if it is to change then he will need to file for it.
IMHO, any good parent is going to be “nosy” and “mean”. That’s how children get to grow up is by their parents keeping them safe and protected. Giving a little freedom is alright but it should be earned instead of expected. I understand that a teenager is going to be defiant and ungrateful but this is where parents get all the “glory”. Sadly, you will have to be the bad guy. A judge would look at you being “in his business” as a good thing, as would a lot of other parents.
I’m not suggesting that every parent doesn’t deserve the chance to be with their children and work out the parent/child relationship but if there are this many problems already what will he be like when he starts to drive…It sounds like his father is being more of a friend than a parent.
Document everything. Get copies of school records and notes from teachers. Find out as much as you can and be prepared. He needs to come home. Then you wait and see if his father files for custody. If he does then let it be his burden to show the court why it would be in your son’s best interest to reside with him. Maybe you could try to work out an arrangement with his father for him to have more time while you retain primary physical custody. This would show the courts that you are willing to work with him but not willing to jeopardize your son’s well being just to make him happy. You are the parent and it’s up to you to make the decision that you think is best for him. Though he’s 15 he’s still too young to make decisions like this himself, especially for the reasons he has given you…