Awww man. I'm sorry. :( The key is it doesn't bother Bun-Bun, and he's still happy and active.
I've dealt with this a lot lately concerning my cat, and the fact that my love for her outstrips my bank account by quite a bit, while she has contracted several mystery ailments. What might give you solace is considering how great his life has been with you, compared to if he'd been without you, and if he doesn't seem to be in any pain, just appreciate and pamper him while he's around. It really sucks, though, I'm sorry.
Take it to the OSU College of Veterinary Medicine (small animal clinic). It probably won't be any cheaper, but they're more likely to know what the problem is.