Hi Brightside, i'd go with your vets advice and ditch the catmilk,it has zero nutrition and is doing him no favors,water will be better for him and he will be fine on that
Sardines are a cheap,nutritional meal for him and if he's a bit under the weather will be easy and light for him to eat,or some chicken minced up, some white fish,tuna,these are all better nutrition wise for him and cheaper than pouches and again easy and light for him to eat and digest
Good luck and hope we've helped,hope he gets well again asap