By "visitation agreement" do you mean a parenting plan that was signed by both of you and filed with the court? If so, that document is now a court order and is enforcable as such. Until a modification has been legally filed with the court (which should happen considering the change in circumstances) the current document stands.
The wording on the document doesn't change simply because he moved. And he doesn't get to interpret grey areas, if any exist. You can let him know that obviously the parenting agreement needs to be updated and perhaps first try talking it over with him before moving on to mediation. If you do decide to be flexible about the winter break (because it's likely too late to get anything worked out in court beforehand), I suggest getting any temporary changes to the parenting schedule in writing via an agreement between the two of you that he can take the kids from x to x and will return them at x time. Define travel plans, etc. Both sign it, maybe get a witness depending on your level of trust with one another.