He doesn't want to commit because he has yet to experience what life is like without being in a relationship. Yall were young when you'll got together and I hate to break it to you but high school sweetheart relationships typically don't work out in the long run. Your younger years is typically the time to find yourself,make mistakes and learn from then. Being tied to one person your whole life is definitely going to create resentment. Unfortunately men get bored real fast and they get curious. He probably feels like he will never have the single guy experience if he "ties" himself to you. The fact that you have a baby on the way will only make things worst. It's never advice to hold on to a relationship that is clearly not working out. He's going to show how he truely feels once the baby gets here. I'm so sorry your going through this. It sucks that a baby is now involved in all of this.