Your dog doesn’t understand that you are going on a holiday!
All he knows is that you have put him in a strange place with strange people and dogs. This is alot to take for a dog who stayed at home his whole life. If your dog is sociable, resilient and adapts quickly, he will settle quite quickly. But what if you have a sensitive dog?
Socialise your dog
Bring your dog out frequently to experience different things, especially to meet new dogs. Make these experience positive so that he knows that every experience you put him to is great.
Get used to handler
Bring your dog to meet the handlers prior to boarding. This will build familiarity and reduce stress.
Bring his favourite treats along so that the handler can give it to him and make him feel better.
Check in before evening walk and dinner
The walk and food will make him realise that this is a good place, not a horror movie. Use treats that he is familiar with so that he wont get diarrhea.