For some pessimistic guys, all it takes is one betrayal of trust to believe that all women are the same way.
He’ll find himself second guessing everything his future girlfriends do or say.
He never stops to realize that by never trusting her in the first place, she felt compelled to betray him to get back at him.
Of course, she is not blameless, but she might not have really acted on it has she not had the idea pounded into her head.
The issue with this type of problem with trust is that there is very little you can do to make him know that you are trustworthy. If he’s never given a chance to talk extensively about how the issues of trust with his family, he’ll never lose that little voice in his head telling him you cant be trusted.