Diana,
I too can speak from the experience of having 3 grown sons, and I'm going to say basically what others have said. And really, it goes for daughters too.
Once they're nearing 20, (and often much younger) in most cases to tell them what a great opportunity this is AND to go ahead and book it all up and ring his boss is likely to be the biggest turn off you could imagine. Even before I read what the others said I was thining, this young man does know what he wants...he wants to stay where he is and do his own thing...or at least, he doesn't want you or anyone to tell him what to do any more.
Have you asked him what he wants to do? I think, if it was me, I'd sit down and say you feel you've been a biut interfering. I'd apologise, but explain that it was such an opportunity and that you got all excited for him and just couldn't stop yourself going ahead and booking the ticket as a surprise for him. And then I'd ask how he feels about it.
What did the boss actually say to you? I mean, did he cave in and say ok, or was he a bit worried about how he was going to manage? I've had sons who've done their own thing. One walked out of higher education at about that age. For some time he did what seemed like very ordinary jobs, but then through sheer hard work he got noticed and before we knew it he had a very interesting job indeed. He's done incredibly well for himself.
Boys are often very late starters, or late developers. They may be a long way into their twenties, even approaching 30, before they see clearly what they want to do. The very worst thing you can do is push. The best thing you can do is support, and always be there and willing to listen. Don't offer advice until its clear that it's wanted.
Believe me, that approach can be hard at first, but it is well worth it when they ring you up and just want to talk to Mum and get your input about something tricky, as my oldest son did this evening.