Ok, I decided to tell him, so I just sent him an email a minute ago. The email said:

Hi Steve,

You asked me a direct question today about my affect looking flat, and I feel now that I should explain more than I did. I have bipolar disorder, which I am well treated for, but as a result of this my affect tends to look really flat. I am self-conscious about it, but I don't think there is anything that can be done. It is something I am still looking into.

I wanted to be completely truthful with you since you asked directly. I think my own experiences sometimes give me insight into what some of the youth may going through. I definitely practice from a Recovery model and believe recovery is possible and expected. I hope that explains things a little better. Please hold this in confidence.

Thanks,

So, now it's done. Hopefully this was not a mistake. Thanks for input everyone.