I’d definitely arrange a meeting with all four of you attending (or five to include the dad host). Keep it lighthearted, but focussed. Meetings, formal communication, lists, pros & cons discussions, contingency plans- these are what grownups do to flesh out proposals.
Have your questions ready, have your boundaries clearly figured out (ie sleeping arrangements, adrenaline sports, medical proxies).
Figure out the full budget for DS with everyone’s input. As the $$$ start to skyrocket (tix, insurance, passport, visas, daily spending money) your DS will soon get the picture, without being able to cast you as the ogre.
If the trip should turn out to be unviable (money) or the discussion hits a deal-breaker then everyone will hear and understand it in one go. You might still get tagged a helicopter mum, but it’ll cut down on the Chinese whispers that are currently happening.
I’m reminded of my mum always saying to me ‘There’s no invitation on the table if I haven’t heard it directly from another adult. There’s just a rumour of an invitation’.
Actually, that said, I’d get my DS to set up the meeting. Since he’s so keen and growed up and all.