A urine sample will only show part of what the vet needs to know about kidney issues. As Ela says, bloods are probably the only way they will know how the meds are working - it is important, as it could show he needs a higher or lower dose, and also as Ela says, it is a prescription med, so the vet has to have certain things to be able to continue to prescribe. I dont know how feasible it is to do bloods from a home visit, it might be tricky being able to get the cat at the right height, and depending on how close to the vets you are, whether storage and transportation is an issue.