It's time you grew up! Not only did you not apply safe sex when dating, now this young man is the father of your child and you've decided you love your ex! What makes you think your ex wants you back and if you're pregnant it's doubtful he'll want the responsibility of someones child. To carry another man's child when you professed your love to him and then race off to your ex is not high on a man's list and they find it hard to cope with and who could blame them.
The father of your child has every right to support his child and visit with his child. You will or have already broken this young man's heart. It appears that you are thinking about no one but yourself and that includes not thinking of the guy you are with or your baby. "Ex" means it's over!
Time to stand on your own two feet and forget the "Ex" and if you don't love the father of your baby then be honest, but be mature enough to let him be part of the baby's life and also mature enough to bring a healthy baby into the world, look after that baby and learn some independence before you rush off into another love affair!
Well, you've got yourself in a pickle. The best advice is to be honest with the father and make sure he understands that this is your decision. Be sure to let the father know that he is going to have relationship with his child even if the relationship between the two of you is no longer there. Good luck.