Your boyfriend sounds like he is sure about what he wants out of life and he should be respected for it, just as you should be respected for what you want to do, which is admirable. I don't think either one of you is being selfish, but only sure about yourselves and your individual needs.
If children are in your future but not his, then it is time to end the relationship with him so you both can get what you want. There are a lot of men out there who would love to have kids, fostering or otherwise. It is not fair to expect your boyfriend to have the exact same life expectations and goals as you do, especially since you changed your mind about having children. The fostering is not the point. You still have to raise the child and care for it in every way. The responsibilities are just as great, if not greater, when you foster than if you have your own children.
You said you both agreed some time ago that you don't want kids. You changed your mind. That is fine, but don't look down on your boyfriend if he does not agree. Move on to make the life for yourself that you want.