Welcome to YT!!!!!
the change in food could definatly be the cause.....next time, try to slowly change his food. for example start mixing it in with his old food and little by little add more and more of the new food. also, i'd love to see pics of your baby whenever you get a chance!!!!! good luck, and let us know what the vet says??
oh also, you dont want him to get dehydrated, so i would give him a little pedialtye to help keep him hydrated, or at least make sure he is drinking lots of water. is he eating??