CUt of allowance and priveliges if he doesn't clean his room up! No need to go into (gory) details just give him a bucket with his own cleaning supplies, paper towels etc. and tell him that he needs to practice getting ready for college when he has a room mate.
Instead of confronting him this time, the first thing you could do is to put a box of tissues in his room, he'll (hopefully) straight away realise that he can use them instead of doing what he is currently doing. Tell him he is in-charge of emptying it from now on (yes i know, telling this to a teenager is like pulling teeth!). Also encourage him to open his bedroom window each day, although if he doesn't listen, buy a small smelly fragrance item that can sit on his table or under his bed etc to help get rid of the smell. These are some things you can do without actually confronting him, hopefully he will catch on. However, if it doesn't seem to work, you may have to tell him straight that he needs to find other ways 'skinning the snake', and your no longer going to clean it up. :)