Originally Posted by smrobs
Since this issue has suddenly gotten worse, I would guess he either has a problem with being worked and gets nervous about it, or he could possibly be having some pain issues somewhere..
Or the third option is just that he's learned he can get away with it, or possibly even been encouraged to continue.
I'm not discounting your options. They are very real possibilities that need to be considered. But I just wanted to suggest an alternative.
I've seen situations where a horse starts getting mouthy, and the owner at first ignores it, so it increases.
The worst is when the owner encourages it (Oh look, he's leading himself! How cute! Good boy). Before you know it, he's got his mouth on everything.