How can I help my boyfriend resist the urge to cheat?

I know it sounds bad but it's not how it sounds. I'm in love with a man who has had many sexual partners and not as many relationships. He was not a committed person. We met and fell in love. He's An amazing man, he's not mean or abusive, he has great manners and is extremely caring and loving. But he's a cheater. At the beginning of our relationship we weren't very serious and he kissed a girl friend of his which didn't come out till around a year. After over a year of being together we were having problems that drove me to another man emotionally and him to another woman physically. We worked it out and were extremely happy. We're expecting our first baby together in two months and he's been amazing. He listens to my needs, if I'm not up for sex he's fine with that. We've always been on the same page sexually. And he's so far an amazing father. He kisses my belly and talks to his son, he works hard to support us. He's a truly amazing person. However recently (over two years of being together) I saw something on his phone and asked to see it. He gave me it without hesitation (he use to hide his stuff) and I found around three conversations with different people of him being sexual, including one group chat. He admitted he asked someone for a video and I saw pictures a woman had sent him. We both had a good cry and talked about it, everything was deleted. I asked him why and he seemed so truly unsure. I love him, we have an amazing relationship and this has been our only problem. I will not leave him. Instead I'm asking what I can do to help him. I feel like he may have a psychological problem with needing to feel needed. He had a horrible background with his birth parents abandoning him and multiple women hurting him horribly. He's more than willing to work this out so he doesn't do this anymore. He wants to be a good daddy, it's his biggest fear of our son seeing this. He wants to get married but we're both scared because of this problem. Does anyone know any way I can help him??