It was rude and inconsiderate for your son and DIL to cancel so late.
On a separate but related issue, family holidays organised by Grandparents can be interpreted differently by each of the involved parties. My inlaws organised a very similar family holiday to celebrate a milestone birthday. They paid for their three kids and partners and grandchildren to go. We all attended, but my husband, myself, and our teenage children did not enjoy ourselves. The location, accommodation, food, and activities were all chosen by our inlaws. This is fair enough because they were paying, but none of them were things that we would have chosen for ourselves. We also had to cancel our usual weekend activities to attend, and although there is no bad blood between anyone, my husband and I do not share many interests with the rest of his family. The cousins don't share interests either. My inlaws had a wonderful time which we are happy about, but they talked about the weekend as something special that they were doing for us. From our perspective, however, it was something that we did for them. I am not saying this to excuse the behaviour of OP's son (he did the wrong thing). I am only saying this to point out that parents and their adult children may perceive the same situation quite differently.