Chicken would be better than fish as it has less magnesium in it. Its the Mg that makes up alot of crystals, if struvite is what he has?
Its better he eats some food then none
If he really isnt enjoying the diet then you can try using supplements in his food to acidify his urine and dissolve the crystals instead. Most vets will give the diet but they dont see how unhappy the cats can get. And stress can contribute to bladder issues
Supplements:
Methogel gel
Ascobic acid (vitamin C)
It seems so silly to me to feed a cat who needs dilute urine, biscuits!
Have they xrayed him to make sure theres nothing huge in there that needs removing?