[@jasonwolf] Put simply, he's bored. He's been alive since the 15th or 16th century, studying magic and alchemy for a way to restore his body back to normal, with little success- the modifications he was forced to make were... extensive. He has a lot of experience and not much to do with it, so working with W&R would give him a chance to use what he's learned in a productive manner, and maybe learn something new in the process. 

Long story short, if he's gonna live forever, he might as well make himself useful.