He could have a problem with foods .Do you feed him any people food? What dry food do you use? I know how to do it but I don"t feel comfortable doing it .I have seen problems with owners doing it(was a vet tech) I still tale Merlin in once a year with his shots. But if your shown by your vet several times and have him teach you its not bad (except the oder) Also how old is you dog? age can play part of it.