From experience, my now ex-boyfriend and myself were very close, very trusting of each other, loved each other etc, but he would not let me go to Ibiza for 4 days with my friends, all early 20s (one of my friends lived out there and we were visiting and obviously partying). My friends even came round to see him and said that I worshipped the ground he walked on and would never cheat and they would look after me etc but he wouldn't let up, said he didn't trust other people out there. Needless to say, I didn't go out of fear of going and then my relationship being ruined... he then broke up with me weeks later stating 'I wasn't in the same mind frame as him as I didn't want to go on holidays with my friends'. I laugh about it now, but regret that whilst my friends were out having the time of their lives I chose a boy over such an experience.
If your holiday has been planned, talked about etc for such a long time then, honestly, if he's not letting her go she needs to let go of the relationship and have the holiday, especially if he's showing doubts. I wish I'd known better.