A cousin of mine married a guy who has been adamant about not wanting children at all. He was clear on this from the beginning. She had always wanted children but made the huge compromise to not had them, and although they are still together decades later she has such a sad look as all her siblings have gone onto have kids. She obviously went into this knowing the deal and that is her call.
Just tell the truth that if the guy is unbending on having children you can’t make that compromise yourself. You won’t be happy without them, he won’t be happy with them. That is all you can do, I wish you well.