The link KAK posted is very informative, and Fister has been through this years ago, so I glad you decided to take him for an X-ray. It really is painful and can be fatal if he's not treated in time.

What makes the vet think that a diet will solve it? That could take ages. He should at least be flushed through!

Fister and I are sending lots and lots of positive thoughts for Pouncer.