Agree with the other posters re: adding supplements and limiting activity. Maybe not eliminate running entirely, but try to cut back on the frequency, especially anything that would involve twisting. Regular exercise (and keeping weight down) is very helpful in managing arthritis, but you want it to be lower impact, such as walking or swimming. I do understand about not wanting to steal his joy though ....
Also wanted to add that Mulder has been on Rimadyl for about a year now, with no problems. Scully took Deramaxx for several years, also uneventfully. Neither drug seemed to make them spacey, although Scully was eventually put on Tramadol in addition, and that one can make them a bit sleepy.