The first thing you need to know is that you shouldn't be making a decision based on what is best for your son. You need to do what's best for you! If you are happy, then your son will be happy in the long run. If you make a choice that is good for your son, but bad for you, your misery will end up harming your son.
What's more, if you make the choice to go back with your ex. , you will harm your son in yet another way. Going back to someone who mistreats you and your son would be a masochistic and self-destructive act. Your self-destructive behavior could result in two bad outcomes: 1) It could invite your son to model himself after you, so that he would learn to become a selfless masochist himself; and/or 2) it could teach him to disrespect women. After all, if his own mother doesn't respect herself, then why should he respect women?
Make the choice that 's right for you and be happy. When you choose to be with this other man, you are not depriving your son of contact with his biological father. His father can see him any time he wants.
Don't be surprised if his father doesn't follow through and visit his son. From what you've said, he's the kind of man who wants what he can't have. So long as you were available he wasn't interested. Now that you want someone else, he's on his knees begging. But his best behavior won't last long. Soon he'll be too busy for his son all over again.
If your new guy is as nice as he sounds, your son may end up developing a special relationship with this man. Oh, and if your son gets really attached to this other man, your ex. might just show some interest in your son. . . once again because someone else wants him! Enjoy your life. You only get one ticket to ride.