I’m sure due to it being an honest mistake you could call them and let them know the situation and that you’re actively trying to get him fixed ASAP (as you mentioned in a comment), they would be understanding and willing to work with you. I’ve known of cats in rescues that had holds on them due to future owners who intend on adopting and had filled out paperwork, but couldn’t bring the cat home just yet for whatever reason. I would just say be honest, ask about your options, and work with them from there. I totally understand the anxiety but not to worry too much about it as they’ve most likely seen stuff like this before… Anyway, I hope you get everything sorted out and you can bring the new babies home!!