That's helpful that he has already disclosed that much to you. This is how I would handle this, IMO. You could casually bring it up and say something like, "I've been doing some reading...Do you think it might be bipolar disorder?" - giving him the opening. Then if he says yes, you can say you thought it might be but wasn't sure, etc, etc. If he's still hesitant, let him know that if it is bipolar, it's something you two can handle and that you're not going anywhere. The key is to ask the leading questions and allow him to agree. Even if you are sure, approach it with a sense of uncertainty, but concern. Again, IMO. Others may prefer a more direct approach.