My husband and I have a blended family. I have two kids and he has two kids, we brought both his kids to live with us because they were getting bad grades and doing bad things. His son who was 13 at the time, addicted to porn and didn't have any rules. I set rules and monitored him. It was me because his dad didn't know about computers or online grades and cell phones. But this made me the bad guy. His son was not happy so it made my husband not happy. 4 years later he decides to leave for his kids happiness. Are you kidding. So I sell the house move in with parents and I'm still talking to him. He sends miserable and said he made the worst decision of his life. I'm keeping my distance but think there is so much love that maybe in the future after the kids are gone we can have our chance. I don't know if this is realistic though or healthy. Could I trust that he wouldn't leave again.

I don't see this as a matter of trust. He didn't lie or cheat behind your back; he just made a choice of his children over his spouse. Some people put the kids first and some put the new spouse first. There's no right or wrong. And you married him knowing that he had a problem child.
Sure, give him a cautious chance. It sounds like he really does love you.
Just try very, very hard to communicate all your feelings - both of you - before they escalate to breaking up.
Yes I know, easy for me to say. No one ever said it's easy.
Chances might be very good for happiness, with the kids grown.