Does you son have the insight to know that divorces often lead to problems between ex spouses and shyt will be said that in all reality just isn't true?

Maybe sit down and have a chat with your son.. If you think he will be able to understand how something might be said and how to just stay out of that situation? How to gracefully leave a conversation that he in fact has no need to be in.

Is it at all possible that you and your ex can at least agree to not speak ill of each other in front of him?

One thing I learned during my Divorce is I had no control over what my EX promised our daughter and he had a really hard time at first balancing his time between her and his new wife ( yes also the one he cheated on me with) It would tear me up to have my 6 year old daughter come home crying because he didn't take her to the beach or McDonalds etc.... But in the end things did work out.. She got older and was able to verbalize to him how his actions hurt her etc.

Just make sure that you tell him how much you love him and to enjoy the trip as much as possible.

I would suggest that you make plans for yourself while your son is gone to keep yourself busy and not solely focused on how his trip is going.