You just have to stay strong for the little man growing inside of you. He does need to know his father if the father is willing, even if it doesn't seem like he does much for him. Don't let yourself be the blame of a bad relationship.
My boyfriend's mom was cheated on 3 times by his father, and yet, as much as it hurt her, she stayed strong for her son and kept in contact with his father for his sake. Your son's father will hopefully grow up and mature one day and get over this stage. You never know what can happen! You may meet someone between now and the end of your pregnancy and fall in love with another man and forget all about your ex. Just stay strong, positive, and focus on your new baby to come.
Congratulations and good luck!