Hello and thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 25 years experience and would be happy work with you.
I am not sure what advice you are looking for, but it is possible he was exposed to something that is making him not feel well. Vaccine are not the end all protection and if he picked up a "bug" while there, he may be reacting to it. If it was something he was vaccinated for, he may have been "sicker" without the vaccine on board.
It is possible that he is just tired and need a few days home to adjust, but if not, he will definitely need veterinary intervention to get to a diagnosis and have appropriate treatment and supportive care prescribed. I am sorry that I can't be more specific, but when dogs are in contact with large numbers of other dogs, diseases like kennel cough or canine flu are a real concern. Try to keep him eating and drinking as best you can and give him a few days to let his immune system kick in. If he starts to get worse however, I would have him seen sooner.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.
My posted replies are for general education only and not meant as a diagnosis. Only after a thorough veterinary examination can a diagnosis for your pet be made and specific treatments be advised or medications be prescribed.