Most Helpful Guy
The longer your boyfriend holds out on telling the truth, the more painful it'll be for him to hold in that information. If anything, revealing this truth can possibly give your boyfriend and your father a bonding moment together as they can relate to the crimes they committed! If you know for a fact that your boyfriend wouldn't attempt a crime like that again, just be open and honest with your parents. A mature person with a plan of action is always comforting for the parents when they believe in their child's decisions.
One way or another, the truth will have to come out. Why not get it over with to put some ease on your boyfriend's mind? If your parents aren't accepting him, just explain why your mother is still with your father, despite the crime he committed. You love your boyfriend very much and would do anything for him, no matter his past! I hope this situation will get resolved for the both of you so you can be happy in your relationship together, knowing your parents will accept him as part of the family!