Was an x-ray taken at his last visit? If so, was there evidence of skeletal MBD? If not, then with his going down it is going to be very stressful to both transport and get an x-ray in his present condition. With your vet thinking the cause is Kidney failure, I would treat him for this condition.
Please be aware that up to, "Late stage" kidney failure or disease as it is also known; according to the rat research, the kidneys can improve with treatment. (see narrative by the research veterinarian posted below)
For reason that kidney disease being an age related progressive increase in phosphorus retention by the kidneys and with the also age related reduction in calcium bone retention; to address this condition would require using the MBD treatment, save with Calcium citrate instead of Calcium carbonate, because Calcium citrate was found to improve lowered kidney function in rats, but Calcium carbonate also tested did not improve kidney function. With this treatment, if he begins to per up you will know you have hit the target.
This is an excerpt from a researcher vet's journal on this issue that is a bit more technical, but not a lab journal:
(This article is a narrative by a veterinarian on Kidney Disease in Rats, which explains how kidney disease develops, and ways to improve it; not a lab journal.)
https://vetspace.2ndchance.info/chro...-your-pet-rat/
Secondary to the MBD treatment, you can try giving him yogurt thinned with a bit of water and pulled up with a feeding syringe to give slowly at a 45 degree angle to the side of the face with the tip just inside the lip. Using this has brought back squirrels that have gone down to restore them when they stop eating and or reduced in weight.
Note To prevent the earlier onset of kidney disease, lower the amount of the diet by a bit every couple of years, and likewise increase the Calcium using Calcium citrate at the same time to bring up the Ca:P ratio a couple of points every couple of years from mid-life on, as this will help to counter the rexcess retention of phosphorus by the kidneys that increases progressively with aging.