Hi. I don’t think you need to worry necessarily about the renal clinic. It’s likely they will take more blood, do a urine test, check your dads weight. They may be able to advise on diet and fluid intake. Of course if someone’s renal function is really bad there would be discussions about the need for dialysis but don’t think your dad is near that yet.
If it helps, my MIL’s eGFR went down from 29 to 23 in December but went up again to 26 after we made a huge effort to get her drinking more. She’s 93 and generally well although sleeps a lot and am sure her kidney function affects her sleepiness. She feels ok in herself though which is great