If your vet is probably right.  My guess is the stress of rehoming and a new
environment probably reactivated a latent Feline Herpesvirus infection
(FHV-1). Your cat doesn't want to eat because he probably can't smell his
food.  Cats won't eat if they can't smell the food.  Try heating up his
canned food to about 90-95F.  Heated food is more aromatic and may help
stimulate his appetite.

You might also want to try steam inhalation to open up his nasal passages.
Bring him the bathroom with you and play with him or groom him while the
steam builds up.  I've found cats are less stressed if they're in the
bathroom with you while the steam builds up than if they're brought into an
already steamed room.  Your presence will also have a calming effect.

You can probably reduce his symptoms by giving him 250 mg L-lysine twice a
day.  L-lysine interferes with herpesvirus replication.  I've had excellent
results with Enisyl-F, its a L-lysine supplement in a paste that's
administered very easily via a preloaded dial-a-dose syringe.  One ml
contains 250 mg L-lysine.  Here's what the syringes look like:
http://www.maxshouse.com/Drugs/Enisyl-F.jpg

Here's more information:  http://66.187.129.130/vetoquinol_pdfs/Enisyl.pdf

Here's the best price:  http://tinyurl.com/bbaqr

Good luck,

Phil