Confront him about it and work it out.
However, don't give up hope just yet. Keep compassion and forgiveness as a main priority before accusations. Don't make him leave, and don't make him feel like he has to. Real love lasts for years upon years, and if you let a relationship that is set to be married die out now, then you're in more than financial trouble.
But make sure your mother is ready to completely accept him for what he's done before talking about it. If a grudge or anger is still held, it won't work out in the end.
Hope that helps.