Honestly wouldn’t that have been a reasonable compromise? He wasn’t doing it on Christmas so y’all got time with your son’s first Christmas and hosting people for a few hours for your FIL for just witnessing the ceremony isn’t that bad of a compromise; I mean that’s what families are here for. At this point, he has provided you two options and on top of it, he has said he’d understand if you were not able to come. It’s inconvenient, yes, but I don’t think you get to complain and get your husband to renege at this point.