OK first of all, I wasn't intending to snoop on his phone but he asked me to set an alarm on his phone and that's when a suspicious text popped up. I was planning on ignoring but I couldn't get it out of my head. So one night I just wanted to find out the context of the messages and that's when I found a ton of messages between him and other men. As well as naked pics sent back and forth. They are about a month and a half old but we were definitely together when it was going on. Should I just let it go?
I am hesitant to say that he has done anything wrong without a bit more background. This is also to give you a different way to look at the situation.
May I ask how old you both are? How long have you known him? How long have you been a couple? How long after seeing the 'suspicious' text did you 'snoop'?
These days so many people rush from just meeting into being in a committed relationship without taking time to actually have a couple of dates or spending time getting to know each other. Some people count the first date as the beginning of being of a monogamous couple. Sometimes individuals have different concepts of when 'being committed to each other' begins. Could this be part of what has occurred in your relationship?
What I am trying to determine is if there was an expectation of commitment to each other at the time of his communications with others. If you had just started seeing each other, then he may not have been as involved in the relationship at the time as you were. Since the communications have apparently stopped (no recent texts,) he may now be at the same point of commitment that you are.
For him, those texts may count as past relationships. Something he was casually pursuing when he met you. Something he has let go to be with you.
Are you interested in being in a relationship with him? Seeing how the emotions and the relationship develop? If so, then explain what happened and discuss what happens next. As much as you might feel betrayed by his texts, he may feel the same about the invasion of privacy. If both of you can work trough this then you may have a stronger relationship. If you can't then it is better to let go before you end up hurting each other more than you may have. Talk with him and listen to him like you want him to listen to you.
Another thing for you to think about is that his past is his past and he cannot change it (you can ask about his past, but if doesn't want to talk, don't try to make him.) If you have issues with what he may or may not have done before getting involved with you, you either need to accept it is part of what makes him the person he is at this point in time or walk away.
Good luck.