First off, I was a bi-polar teen with suicidal tendencies. There were many things in my life that I just could not deal with, particularly at school. A lot of bipolar meds are actually worse than the highs and lows, mostly because you don't get the highs at all and then everything either feels like a low or like you are so increadibly tuned out that you feel nothing. The BEST thing is to channel it all into creative endeavors (like writing or painting during the "blue" period). Also, though he would never tell you, are there problems at school? Maybe suggesting letting him attend somewhere else? He's already going to feel wierd and left out because he's going to a therapist, and he's likely "blocked off" from you, but maybe if you get someone like older bro or a trusted male to talk with him in a casual way (i.e. at a ball game or something) he'll open up. And, as the middle child, if he still tolerates Mom, maybe have some special lunches and go to the movies or something with him. I know it seems like it is all the "disorder", but truely suicide attempts are not usually due to bi-polar disorder, it's usually much more.
(BTW, I have a long history of the disorder in the family as well, and tha majority of us turned out normal and functional.)