It could be allergies...my older yorkies does that. She will sneeze and sneeze and sneeze and sneeze .When the grass has just been cut, If i spray air freshener, when she get nervious. etc. Does he seem to scratch himself alot like he has flees ? There is a web site to check what your vet told you he has-see if the systoms match what your baby has ...good luck i hope he gets to feeling better. keep us posted.
www.ivis.org also check the food he is eating ...i changed food with mine and it seemed to help.