Hi
It could be because his teeth are sore but it might be that he is not used to having different people open his mouth (whilst he is on a table)? Get as many different people as possible to check his mouth, smokers, non-smokers, men, women, people who have been handling garlic and as many different things as you can think of so whatever smells etc are presented he is ready. Some judges almost cover a dogs eyes when examining bite and others can open a dogs mouth by approaching the muzzle from underneath and lifting the top jaw - again try all sorts.
You may have to start with people just handling his muzzle rather than opening his mouth so he gains confidence (and don't forget lots of praise/rewards when he gets it right and silence when he doesn't)
Christine