Why is my cat eating litter with kidney disease? Is any litter safe?

I'm sorry to hear about Oberon's kidney disease! Eating litter is most often a sign of anemia in cats - this would fit with his kidney disease, since anemia is a common finding in late-stage kidney problems like this. It may be worth talking to your vet about treatment options for this problem (such as Epigen injections, or even a blood transfusion), as this is likely going to be the only way to keep him from continuing to eat whatever litter you provide.

In terms of safety, ingesting large amounts of any cat litter can potentially cause some stomach upset, constipation, or even an intestinal blockage - there is no evidence that non-clumping litter is any safer than clumping varieties, or that pine would be any safer than clay. So I would go ahead and continue using whatever Oberon is most comfortable going to the bathroom in, and see if you can treat the underlying anemia to help address the problem that way if possible :)