He would probably die
Especially because he is too thin.
But not as fast as some people may think
Recently, some guy made the stunt of living 2 month with only potatoes. But it was 20 potatoes a day which weigh around 2 kg.
Also, 2 months is not that much. In extreme starvation, normal people survive up to 2 months with only water.
Who needs calories anyway
500 g raw potatoes is about 1600 kJ of nutritional value. Can a man survive 1 year with 25% of basal energy intake?
As mentionned earlier, most people die after 2 month of fasting. Little food can buy you some time, but probably not that much.
BUT... in a case of extreme obesity, a man fasted for 382 day under medical supervision.
He was given some salt, vitamins, yeast and unlimited "non-caloric liquid".
You could imagine that potatoes could give the salt and vitamins intake (not as good, but your scenario allows your test subject to loose sight, teeth and sanity).
However, digesting your own fat needs A LOT of water. 1.5 liters cannot be enough.
Drink your own pee
The role of urine is to get rid of the waste in the blood.
As such, it is not really toxic. Not much bacteria and whatever toxin are in the drunk urine will be filtered (again) by the kidney.
But if you re-inject the toxins, your pee will always get more yellow and at some point, your kidneys will surrender.
Especially if you are fasting as (whatever snake oil seller may claim) you would produce much more toxins.
I'd say that urine would get toxic within 1 week only. If you have a micro filter, you're good.
eat your own poop
What the hell would you expect from that??
Some argue on the Internet that the bacteria would not kill you. But if you eat a significant quantity, the nitrate surely will.
Anyway, poop is expelled because your digestive tract doesn't want it. Keep in mind that digestion needs plenty of calories and water so don't try to digest useless stuff.
In the end
Lock a 200 kg man with 3 l water and 500 g potatoes a day and I'd say he has a good 15% chance of beeing a happy, toothless, blind, crazy winner.
Tell him not to eat his own poop.