You need to take him to a chin experienced veterinarian who can do a thorough oral exam. This includes radiographs as simply looking in the mouth is not enough. There is no "cure" for dental disease. If there are only a few spurs, the vet can file them down. If it is an issue with the roots, nothing can really be done.
The important thing is you get in him to the vet asap because it is very painful and they will stop eating. Dental chins usually die of starvation if left without veterinary care. The vet can give you some options after they figure out the extent of the problems.
Where are you located? If you don't a vet, a member here in your area may be able to point you in the right direction.