Had he not already known what we were doing this might work - but unfortunately - he knows the score :(
If you end up not going, you tell him you're going on holiday and then take him to the zoo or farm park or somewhere else he finds exciting... he honestly won't realise the difference at that age! If you have to, tell him the aeroplane broke down or Majorca was too rainy, or something and you've decided to have a special holiday at home! What ever you do, don't blame those spots! This a white-lie occasion!