I heard back from the vet this morning. Calvin's calcium is abnormally high, but his parathyroid is in the low-normal range, so it's not a parathyroid issue. The vet said we're looking at probable neoplasia (tumor) and likely cancer.
I was given the name and number of a diagnostic specialist in Dallas, but after talking it over with Steven, we're not going that route. We won't treat cancer, if that is what it is, and he's not in any shape to undergo surgery if he has a tumor that needs to be removed.
So, whatever days he has left with us, we'll let him live them peacefully at home without further medical intervention for this new issue. We will continue with his current regimen of pills and hydration, but we're leaving it at that.