So the father-in-law sees himself as intelligent, educated and informed? Lawyers have to become fast experts in obscure areas, I'm surprised he hasn't had to learn about bipolar through his work.

You can say a lot of things about lawyers but one thing they are (or were at one point) is diligent scholars. If you can convince him that you know more about this than he does, I would quote a couple statistics off-hand, it might prompt him to do his own research. He sounds like the kind of person who gets annoyed if there is something they don't know about.

I've gotten into debates with guys like him. If they are good at debating they can usually pretend to have some idea what they are talking about. You throw them off by knowing facts they don't. Do a quick search and stick a couple specific statistics in your pocket. He can't counter because he isn't familiar with the material, then has to acknowledge that you are.