well first CONGRATS AND WELCOME!! second.... you need to find a vet you are comfortable with. and third... i would ask that they run tests on his poop. I woudl NOT start him on antibiotics.. he just got home from what could only have been a very stressful flight. You need to get him eating and peeing and pooping normal. My guess would be it is stress induced. Just make sure he is eating... he might not want to eat his kibble right now because he is adjusting. He is on a quality kibbble (no Eukanuba, Iams or any of that other junk). If you can't get him to eat his food try something like boiled chicken, scrambled egg, cottage cheese, apple, cheerios. Just something so you know his system is working and see how his next poop is. also make sure he is drinking properly.. if that means using an eye dropper to put it in.. make sure he gets it in these next few days. Good luck
I'm sure he is ok but just keep and eye on him for the next couple days... if he develops diarrhea bring him to the vet.