My vet wanted to do an ultrasound on my renal kitty to see if she had any of the very commonly-associated issues/conditions that go along with CRF/CKD in elderly cats.
The ultrasound payed off for me, as it turned out my girl had IBS and had to be put on prednisolone. This has helped her a lot.
I'd feel safe doing the ultrasound on your boy as LONG as you find out from your vet why they want it. It may be to just check his organs in general, to check for IBS, to see if he has any tumors/etc., or for other reasons.
Also, check your kitty's blood pressure and thyroid (T4) levels, as high blood pressure and hyperthyroidism are common conditions that often come along with CRF/CKD. I'm not trying to scare you, but it's worth catching some of these things early :}
As an aside, for renal disease in general, a site that will come in very handy is Tanya's feline renal disease site:
Tanya's Comprehensive Guide to Feline Chronic Kidney Disease - Everything You Need to Know to Help Your Cat
This site has saved my butt and my kitty's several times.
There's also a Yahoo! group that is wonderful. There are a lot of members and if you have ANY questions, they won't hesitate to help you.
Sedation CAN be hard on an older kitty, especially one with renal disease. However, if the vet is aware of it, it's a lot safer. My girl is 17 years old now, and has been sedated many times with zero issues.
BTW, renal disease is NOT a death sentence. Sometimes a vet can scare the crap out of you when they tell you that your cat has lost 75% of their kidney function. But with management, medications, and love, CRF cats can live a LONG time. My girl was 14 when she was diagnosed, and her kidney values have actually gotten better over the last 3 years and she's still going strong!
Feel free to join the Yahoo group, they are wonderfully supportive and are happy to share their experiences and knowledge and have reassured me many times on kidney issues with my girl :}