Horses starting at age 2 and until they are 5 years old need their teeth done yearly. During this time period they are shedding caps (baby teeth), when these caps come off they leave razor sharp points that will cut into the cheaks and tongue. These caps can also become retained, causing extreme pain, pulling these off will releave the pain. The slightest pressure from even the headstall running down the horses cheek with cause discomfort. The other thing that I would make sure was done is that his wolf teeth have been pulled. This is a very small premolar located in front of the upper cheek teeth, tissue hangs down and the bit will bang against that tissue and cause pain. Also at this age they are getting their getting in their K9 teeth they can have 4 of these coming in also.

So all in all I would get your vet out and have his teeth done it is the best place to start and will give him immediate relief. They will sedate him and it normally only takes about 40 minutes for the whole procedure (with motorized stuff) and he wont remember a thing. After the age of 5 years you can go to every other year on floatings as long as motorized floats are used, If the vet still uses hand floats it will still have to be done yearly.

Hope this helps, and let us know how it turns out.