I cheated on my ex-boyfriend and now he wants to know how can he trust his new girlfriend without cheating on her?

You're the reason why people don't believe in love & why people screw over others. They're hurt or scared because of people like you, then others get hurt because of it. He'll probably be hurt by it all his life. His new girlfriend doesn't deserve to be untrusted or cheated on because of YOUR recklessness.