Well, I am responding but have to admit, this is way out of my scope of knowledge. While I do know steroids can trigger mania, I don't know what your options are for your son. He obviously cannot be without his arthritis meds or bipolar meds, so this is a real tough one.I think between the specialist (if he is world renowned you would think he could put that noggin to use and help research a solution for your son) and a psychiatrist, someone should be able to come up with a solution...surely there must be someone out there in the same situation.
Apart from that perhaps google the topic and see if there are any answers out there to bring up with the Dr. I am very curious to see what the Drs say, please keep in touch, and in the meantime I will do some research of my own at the library and see if any research articles come up.
Sorry I couldn't be of any immediate help!
ps-what meds specifically is he on?