I would term him early too to fit your new family. Since Mom intended pulling for her mat leave before baby was born anyway, it isn't a case that she was hoping to get new baby in a routine before leaving your care.
You have to do what suits your business first so even if this child was dream to watch, it seems like letting him go on your schedule would be best.
They were planning to leave Oct-Nov anyway and you can't plan a replacement with such a big time frame. It only reasonable that you plan for him leave earliest and protect your income.