M.P.
http://en.wikipedia.org/wiki/Bipolar_disorder
Hi S.- read the above link, particularly the part titled "Manic Episode". One of my aunts and one of my uncles is bipolar, and they do a lot of what is listed- big spending, drug addiction, grandiose thoughts. Mostly, though, you can just tell when they are having an episode. Does this sound like your husband?
The thing I think is weird is that a doctor diagnosed him and recommended THERAPY. You can't treat mania with therapy, you need medicine. This alone makes me think it's NOT bipolar disorder. I would press for more details from your in-laws. Let them know that if hubby really is bipolar, then he can't see his son unsupervised until he is stable. This makes sense, actually- my aunt and uncle were never violent or anything, but they were really irresponsible and just nutty as a squirrel (seriously, I could tell you some stories).
I hope your husband is NOT bipolar, for the sake of all involved. From where I sit, it sounds like he is going to therapy because he has "issues." If your husband wants you to wait for therapy to fix him, he needs to tell you what therapy is supposed to fix. THEN you can make your decision.
ALSO, ask your lawyer if there is a way to find out what is in your husband's medical records. Good luck.