Young adult DS who still lives at home.
I was going overseas to compete in a sports event the weekend before his birthday. Partly because I fancied some support and partly because he really doesn't need any more "stuff" I said if they wanted to, I'd treat him and his GF to a few days in the beautiful European city as his birthday present, and I paid for flights and accomodation. A lot more than I'd usually spend on his birthday, but as I said, I also had a vested interest. The flights were organised so they'd arrive after me, the day before the event and then have a few days on their own after I left.
They split up a few days before the trip. GF (of 2 years) contacted me to apologise she wouldn't be going and I think would have offered some cash but I told her not to worry and to look after herself.
DS decided he didn't want to come alone, and by this point I had some other support coming too so I didn't need him in the same way. I.e. he wasn't leaving me in the lurch.
We were able to refund the accommodation but not the flights.
I got him only a very token gift to open on the day, as I'd already paid for his unused flights. And if I'm honest, my head was in the event and I had no other gift ideas.
We went out for lunch when I got back.
Do "owe" him a gift? FWIW he's accepted everything and doesn't expect anything.