The book is Tetrasomy Two by Oscar Rossiter.
You have misremembered a few things. It's a novel not a novella, though it's a short novel, and the catatonic is not the protagonist's uncle but just a man called Ernest Peckham.
The bit about the weight of the bowel movements appears when the protagonist is giving a presentation on Peckham:
"The first slide shows the results of laboratory work done on E.P. in this hospital. Notice the dates. There are no entries between his admission and read-mission and then none until recently. There was no reason to order more. He has never had a complication, an infection, or even an elevation of temperature in the twenty-five years he has been here. Notice also how nearly identical these findings are. Either his metabolism has not varied at all or our lab reports the same results on all specimens.
"To eliminate the last possibility, I sent some specimens to a private lab and others I examined myself.
"The next slide, please.
"You see that the results are the same. On the last line are the weights of stools he passed; these all weighed 184 grams.
"The next slide will be the last we will see for a while. This was also prepared in a private laboratory and is a diagram of E.P.'s chromosome count. The circled pair are extra, for E.P. has two more than the usual compliment of forty-six chromosomes. Those duplicated are the No. 2 pair, which give to this abnormality the title "Tetrasomy two.' This has never been reported before, and were it the only unusual finding in this patient would make him worthy of being presented here as well as the subject of an article for the literature."