I see a few possible situations here. When there was talk of swapping days so kiddo could go on the trip, were these the days you offered? Mom could have rethought her position and is doing nothing more than yanking your chain. You'll find out on Sunday. If she still refuses to allow him to go on the trip, you could file against her for contempt. I'd likely just document to present to the court when you go for permanent orders.
It also seems that kiddo will be missing school if he goes on that trip. I may or may not be cool with that (were he my child), depending on his overall attendance, grades, etc. That's really something that you should have discussed with Mom, order or not (and I speak as a divorced parent who did pull my kids from school on occasion...), before planning the trip.