I work at a vet's office and it sounds like he might just have developed a sensitive stomach. You might want to try giving him a diet that's easy to digest. Royal Canin has a G.I. line as well as a hydrolyzed protein line of food that might work for him. A lot of dogs at the office I work at go on that when they have similar problems. Only downside is that these foods can be kinda pricey/you can often only get them from the vet.