EvilEdna, I don't think you should consider having kids with this man until you can come to terms with the fact that he already has children. And it is normal and right that they are his priority in life. That is a good thing that means he is a decent father, which should appeal to you.
This in NO WAY means your future children would be any less precious to him or less exciting.
My children are all as important as each other to me. I am just about to have a second son. He will not be less precious to me because I have a son already. They will be different people and I will adore them both. Love for your children is not finite; it is infinite.
You need to deal with these feelings about your step children or they will ruin a good relationship. If I met a boyfriend who felt like you about my existing children, it would be the relationship with the boyfriend that would go. I don't think I could be with an adult long term who was jealous of my children.
In your shoes, I would choose to try and love my partner's children because if you stay together, they will be your children's siblings. That can be a really special relationship for your own children. See them as part of your family and think about how nice it will be to add to that in future.
I am not trying to make you feel bad at all because I think human beings are complex and we sometimes feel paranoid/jealous and that's life. But it is a good thing to try and move past it because otherwise it is a terribly destructive way to live your life.
Good luck!