If he's still sleeping with his ex, he's not very interested in anything serious with you.
If he's having a baby with his ex, she will always be a part of your life if you were to end up together.
If he, at the time he was with his ex, was telling you he wanted to be with you instead, then he's not an honest person to those he claims to love. What would stop him being with you and having someone else on the side if he's done the same thing before?