I would tell him as much of the truth as possible. My three older daughters all know my dx, one has actually been dx with bp herself. He probably already feels that something is wrong. Sometimes things make more sense when you can put a label on it. After all it's a disorder, not a character flaw. He needs to be made to understand that so that he can see his fathers illness in it's proper light.
It all depends on his maturity level of course. My 10 year old knows I take meds but not what for. He's never asked. I guess when he does I'll tell him the truth.
Good luck to you
Bipolar IIIt is a melancholy of mine own, compounded of many simples, extracted from many objects, and indeed the sundry contemplation of my travels, in which my often rumination wraps me in a most humorous sadness. -- William Shakespeare