My vote is to try now, but with no pressure, and if he's not ready - as others have already said - try after the holiday. If he is ready, you could have it sorted in a few weeks. However, if he is not ready I would abandon the idea very quickly and try again after the holiday.
I RUINED a holiday with my ds2 when he was 2.5 because of potty training. Looking back it was completely laughable. My ds was 100% reliable to pee in the pot when he was bare bottomed, but put a pair of pants on him and we had ten wet pants in a couple of hours! I did not want to `set him back' when we went on holiday, so I persevered althrough the 2 weeks, but it was really grim and he still wasnt dry when we came home!
The `lowlight' of the holiday was when we used to eat out in the evenings and I had to check to see whether the restaurant had padded seats or not - If seats were upholstered I wouldnt let us eat there for fear of accidents etc.. The quality of the food didnt come into it! I bet my dh felt like divorcing me!
With the benefit of hindsight I would never have done this. After all, we'd paid for the holiday - it was meant to be a relaxing time and I ended making myself and my dh (plus, most probably ds) more stressed than usual!
So I guess, if you are not bothered and are unsure - my vote is for definitely afterwards