I think that it is amazing what you said about the horse. I bet that he is so comforting to be around. I envy you too. I just love horses.
I would take it one day at a time with telling him that you are bipolar. When you feel comfortable, then I would talk to him about it. Unless you want to tell him right away. I think that you will know when the time is right and then it will come easy for you to tell him. I have faith that you will know when the time is right.