To me it sounds like you want to tell him. It would be very unprofessional of him to treat you any different if you tell him. It may be beneficial in the future, if you have to take time off. It just comes down to if YOU want to tell him. Personally, I have told most of my employers just in casual conversation. Not ever had a bad experience from it, but when I was going through a real bad depression, it helped to save my job and I think it was because I was up front about it.