My Guinea Pig Has Stopped Eating Hay, Hardly Touches His Vegetables And Has Soft Poos. What Can I Do?

He needs to be examined again by a veterinarian.  The teeth may not be overgrown but he may have an infected tooth root or growth (odontoma).  The stool may be soft due to the lack of fiber in his diet or may be part of the primary problem.  Nausea can also cause lack of appetite.  The gastrointestinal tract of small mammals is very sensitive.  They need hay to help them continue to pass ingesta normally.  If your guinea pig stops eating completely or stops defecating this is an emergency. 
There are veterinarians who specialize in small mammals that you can consult with.