He is at the "teenage" years, so he may be growing longer right
now. Also - purebred oriental-type cats are by definition very
thin, but muscular, cats.
Is his vet worried about how he looks? Or his breeder? Can you
see what his littermates look like?
I would check that first, because as long as he's eating and
playing, most purebred cats take a couple of years to reach
maturity, and will not start "filling out" until they're about
2 (unlike a cat from a colony, who grows fast and reaches full
size by about 11 months - for survival).