If you're reasonably sure you're not dealing with slobbers:
While this rarely happens (at least so I think), get way down under his tongue and check for polyps. Or have the vet thoroughly check his mouth for polyps. Even a thorn, or something similar, that may be embedded in somewhere in the side of the mouth, causing him grief.
My gelding had a polyp waaaay down under his tongue when I bought him as a coming three year old and the vet popped it right off with his thumb.
Then, when this horse was around 17 or 18, he started drooling really bad, so I called the vet out and asked him to pull this horse's tongue out to check if that polyp may have grown back.
It had -- big enough that the horse needed sedated to have it cut out.
Once he healed, the slobbering was gone.
Infected teeth might also be an issue.
