So - to recap, the yonger one does not want to go, and DS16 keeps mum about whether he wants to go or not?
Since you had to renegotiate custody - ALL changes should be agreed with you first, IMO. It is my kosher to get kids involved, he needs to follow the rules as written, to a T. I have learned from experience, and have a 0 tolerance for breaking visitation rules now.
Just sit them both down and discuss - if they say no - too bad, XAH just wasted money on airfare, he will know to run it by you beforehand. If they say yes - still have a talk with XAH regarding how this needs to be handled in the future
I feel like every time I give a little bit of leeway - my hand is getting Bing chopped off in its entirety. So not more
P.S - XAH did the same thing as yours - bailed on one of the days a couple of weeks ago. I now require a schedule of activities/events with appropriate supervision to be e-mailed to me. If he does not e-mail it to me by the end of the day today - no visit. If he does not stick to it - it will be documented. I have no pity for him or his slacker ways