Right now, your dog sees himself as the "top dog" and sees the things that he gets aggressive over are HIS things, not yours.
This would've been an easier thing to train him out of when he was younger because his teeth wouldn't have been too sharp (lol, it's always a plus when training!), and it wouldn't have been too embedded in his mind.
I'm not sure what to do, but this link might help some to understand the behavior:
click!
edit: I've skimmed a lot of sites and they all pretty much say at this point you need professional help, like a behavior specialist or a trainer.

Good luck! Hopefully someone here has some other answers/info for you.