Boyfriend stressing me out with his constant obsessing

My boyfriend and I have been together 6 1/2 years. Long time. I've always know that he is prone to obsessing over stupid senseless things, but he's been really bad lately. He's driving me nuts!!! Right now he's obsessing over a tooth problem that he has. I have the same problem and I am trying to deal with it the best I can until I can afford the dentists recommendations. I don't understand why he can't do the same! He's 3 years older than me and I expect him to be more mature. What really bothers me the most is that he is constantly seeking attention when he obsesses. He wants me to answer the same question over and over, look at the same thing (this time his tooth) over and over, EVERY SINGLE DAY!!! I can't take it anymore! I talk to him, try to make him feel better and less worried, I tell him things he can do to fix what is bothering him, etc etc. Nothing changes. The next day or sometimes even minutes later he is doing it again. I feel like he's not listening to me, he's just wanting reassurance over and over and over. I just can't handle much more. How do I get through to him? What is wrong with him that causes him to do this and keep doing it? I've tried being nice, ignoring him, getting angry...everything. NOTHING works. I am so stressed out because of him and he's being so SELFISH. What can I do? It's not just that he's stressing me out (big time) it's the fact that he ruins everyday things by worring and obsessing. We can't just talk or have fun or go out. It's all about him and what he's worried about at the time. Please, help me through this if you know how to deal with such a problem.I can't take much more.