The book is [Tetrasomy Two][1] by Oscar Rossiter. It was published in 1975.

[![Tetrasomy Two][2]][2]

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_. He is in the care of the protagonist, _Dr. Boyd_, who is a first year resident at the hospital.

As you say Peckham mumbles the names of companies about to do well in the stock market:

>When she had been gone thirteen - a few seconds, Mr. Peckham said, "Hercules thirty-four," very distinctly.  
>...  
>The stock in a company by that name had been actively traded on the exchange the past few days and had had a phenomenal rise in price. An investigation had been promised because it had started up two days before the award of an important government contract.
>
>I checked the dates again. When Mr. Peckham's message had come, the stock could have been bought at thirty-four. A day later, the flurry in trading had begun and the price had started up.

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."


  [1]: https://www.goodreads.com/en/book/show/2575540
  [2]: https://i.sstatic.net/NJl7o.jpg