What if he wants children?....

To be honest, what you need to do is lay it out. He has all the time in the world to make a decision based on his own fertility. But you need to make it clear that you are very excited to be getting surgery done to permanently remove the possibility of having kids. He needs to understand that this isn't something that might change in a year or two or five and that you are actively seeking to medically forbid it as such (which you are well within your rights to do).

If he realises he will never physically be able to have children with you, I imagine he'll decide either way. But as others have said, generally if he's unsure of the answer by 29, the answer is "Yes, I want kids, but not yet".