I never had a problem with the ascorbic acid powder burning my throat [that I remember (but that's been at least 16 years ago)]. If it were me, I would make sure to dilute the powder in plenty of water [if it "burned" my throat]. I would think [potentially] the "burning" of the mucus membrane in his esophagus would not be a good thing [although I would think it would heal itself relatively quickly].
Sodium ascorbate is typically made by mixing ascorbic acid with sodium bicarbonate, i.e. baking soda, at a proportion of anywhere from 2 to 3 parts bicarbonate to 3 parts acid. From my perspective, if taking it orally, anywhere in there will work fine [but again, from my perspective, the less bicarbonate the better].
It sounds like he certainly did reach his Bowel Tolerance. From my perspective, since his health is poor [and he is older] he will probably have difficulty raising his Bowel Tolerance high [and fast] enough to effectively deal with his health issue(s). If it were me, and I was "fighting for my life" I would definitely persevere in trying to raise my Bowel Tolerance, i.e. ingest all I could, 24/7, i.e. to Bowel Tolerance constantly. If it were me, and it was available (and I could afford it), I would certainly consider intravenous sodium ascorbate [given the severity of this health situation].
From my perspective, Piritta, the "diarrhea" he is experiencing, although inconvenient, is [probably] a good thing [healthwise], at least in the short run [of a few weeks]. The "stool" is watery if there is no remaining fecal matter in the colon to expel. My experience has been that [even with the tablets (vs. the powder)] the reaction can be pretty fast, i.e. the tablets seem to dissolve within minutes. If in powder form, yes, almost immediately, sometimes. And yes, a dose may not come out all in one bowel movement [in my experience], i.e. sometimes several.
I do not understand what you are asking....how come he could take the same amount of ascorbic acid as tablets but not as powder?
If he wants to ingest the same amount of antioxidant, he would need to take twice as much sodium ascorbate as ascorbic acid.
All this is just my viewpoint, Piritta.
I wish the best of success to you and your father!
Sincerely,
David