I want to inform him in a way that causes the least amount of pain if possible.
I think it's good to be considerate of his feelings.
Make it factual and offer what evidence you have.
When I called the OBS (the wife) - I did not "volunteer" information to her, only stated the facts, that her husband had been involved with my wife for several months.
I told her that I was sorry for the pain and suffering this would cause her but felt like it was the right thing to do, and it was what I would want from the other BS were the situation in reverse.