I think MIE has made some excellent points on what it could be. To that all I will add, it when was his mouth checked last. Perhaps he has some hooks or sharp points that are causing problems.
If he has only started doing this , but has always been a good boy, then you have to take a hard look at what could be doing this. I seriously doubt that he is just now deciding to be stubborn where he has always been a good boy. Horses don't wake up and decide, hmmmm I think I'll do blank to irritate my rider. I'm not being sarcastic, but please consider what MIE and I have suggested .
A good horse wants to please, unless he is being given a reason not to.
Be the kind of woman that when your feet hit the floor each morning the devil says, "Oh crap, she's up!".