I started out wanting 4 children. I now have 3 and I have reached my limit. My philosophy is now this: a person can't know if they want 2 children until they have 1; and a person can't know if they want 3 children until they have 2; and a person can't know if they want 4 children until they have 3. So, in his defense, how could he possibly know years ago that 2 children would be enough for him until he started having them?
Second, I have a wonderful, supportive husband who is a caring and fully engaged father. I couldn't imagine parenting my 3 children without his full support. I would be very concerned about losing his support by forcing him to have more children than he is comfortable with.
And finally, the risk of having your tubes tied compared to the risk of a vasectomy for him are much higher. I agree with the other responder that if you decide that you are done, he is the one who should have his tubes tied. He needs to get educated on the topic and learn that a vasectomy does not in any way affect his, um, manhood. :)