You felt like you had a good reason for not telling him, didn't you? So, if he is upset about it, then explain it to him. You might say you didn't want him to be worried about you, etc.

Bipolar is a disorder that's almost always treated by medications. It's not a disorder that you have caused. It's often compared to diabetes.

As we get older, we don't need to tell our parents everything. My mom is elderly and I don't want her to get really upset about things, so I don't always share with her.

I know you're upset that you were told on, but you did what you thought was correct.