Just wondered what food have the vets given? Does your dog like vegetables, could you make a stock to mix with the food even with a tiny bit of low fat meat, like venison or fish? Sorry if not much help, good advice given here already, check with your vet first and maybe you've tried it already(know a little of what you're going through), hope he gets better soon.

Its Hills z/d Hes not allowed anything only this food

You could ask the vet regarding Waltham's Canine Sensitivity control vet diet (Truda was on this one) if he's not eating this one.

I had the same problem with my Moo-Moo who is prone to struvite crystals in his urine and has to be on vet diet for rest of his life.
He was first put on Waltham S/O, he ate it for a little while then went right of it. Switched him to Hills and we haven't looked back. Seems to be a little more flavour in this brand.