its good that you found the mail, because now your relation can change from a relation behind a mask to an honest one.
make sure he can trust you enough (thats one very big problem with sexually abused survivors)to always be able to tell you the truth. leaving the elephant unsaid in the room wont help anyone.
you dont say what you have found, but if you have found something illegal such as child porn, you need to consider that you might make yourself criminal if you dont report it. if you just found a few perverted mails, you need to make him understand that you love him, because its something he has learned so very early and he has never had a real chance to develop normally. in the end it comes all down to one thing:
a survivor will keep his mask on until he is basically forced to put it down. my husband had to lose me first before he could come up with his own truth of everything he had gone through and everything he had done sofar. there wont be an easy time for you in the future, but if he is worth you, then help him, support him...thats what he never had from the time of abuse or before. he is a human and deserves that then. be strong, you will need it.
good luck, take care,
everything is always okay in the end, if it's not, then it's not the end