My fiance goes from hot to cold (behaviorally)

Me and my fiance have been together for almost four years. We have a two year old and one on the way. After my son turned one he cheated on me with a fellow worker. We were only apart for a few months before getting back together. Since then I've tried to trust him and not smother him with questions or accusations. I have recently noticed that he goes from super lovey to nothing. No kiss goodbye. Or anything. He says he is happy about our up coming baby but dont want to talk about names or anything. He also has been staying late at work. He says they keep making mistakes and he stays after to fix it. I dont know what to do.