The reading of his emails was wrong as well, it would be hard to bring up to him unless he knows you have access to it. You should sit down maturely and tell him that you are sorry that you saw it, but you need to discuss why he did what he did. As angry as you could be come across as if it was not a big deal because he is probably extremely embarrassed and my explode into an argument if he finds out you were in his emails. Ask him if there is a reason to why he felt he needed to do that, and that it hurt you. What he did was very wrong but he may feel like that since it was not physically that it does not count. Some people have strange views on these things. You have to talk to him about it but be prepared for him getting angry at you violating his privacy. I wish you all the best and hope you both sort this out!
Best Wishes