So it's hard to tell if he just doesnt like making permanent changes to his body, if he is actually a fencesitter, or if he's actually been lying this whole time and already knows he wants kids.

I went through this with my fiancee and it turned out that he just had apprehension about his vasectomy that the doctor said is normal. The doctor said that no matter how much you know you are deciding against something (in this case, children), it's normal to feel a little panic when the option is completely taken away from you and you can no longer choose it even if you were never going to choose it in the first place. Some people on this sub have also gone though those feelings when getting their tubes tied, same way people on this sub have felt some sadness to learn they are infertile even though they know they are CF.

It's human nature to want to 'keep your options open' even when you're already decided. Humans like having choices, even if their minds are already completely made up.