I think if you say that you are sorry all over again for how this might hurt him and sorry that you have to bring it up at all before you tell him what is up, he will be very understanding. He’ll appreciate that you had an opportunity to hide something from him, but were open, instead. He’ll appreciate that you could have done what is easiest for yourself, but decided to do what he wants, instead. He’ll appreciate that you still see him as the innocent party and deserving of the right to decide how to handle the situation in a way that suits him. This will let him know that he can indeed trust you and that you really are ready to bear the burden for what was your offense. Maybe he won’t handle this well no matter what you do, because that is always a possibility, but I think that continuing your policy of openness is the best way to go.