I agree - let her be the one to tell the child that the trip is off and there will be no more family trips. I know it will be painful but it's not right to give such a young child any sort of false hope when a divorce is happening. And it's even WORSE to let the WAS think for a f'ing second (please pardon me, I can hear my Gran saying "citygirl, LANGUAGE!) that ya'll are pals. Have mercy!
Do something super fabulous with your son and while I don't think giving too many details is the way to go, spell things out IN FRONT OF YOUR W to your son in a way you think he can handle it.
You know what? It doesn't feel like it now but you will find somebody who is far more suitable for an adult R. I am flabbergasted (and trust me, it takes quite a bit, lol!) that your W would think ya'll are going to Disney after she serves you D papers at work. I had to read that about 10 times to be sure I wasn't misreading.
To anybody reading this.. if you are SERVED divorce papers do not go on a trip with your WAS. It doesn't matter if it is "for the kids" or the trip has been planned for 25 years. NO NO NO!
Make sure ANY and ALL perks of marriage cease to exist.