If after your trip to the vets, you are just left with a very hungry cocker, then I have heard vegetables help fill them up without adding to their girth.
I am sure I have seen it discussed on here someware, so you could do a search. Saffi's mum was also on a part vegetable diet at the suggestion of a vet, because of a similar problem. Saffi is so far (6months old) not showing any such tendency and indeed is a bit of a fussy eater so I have no first hand experience.