Thanks for your query.
Consumption of toilet acid is an Emergency, it is considered as Acute Poisoning
unless the quantity consumed is very minimal and negligible.
In such an emergency you need to avail services in a nearby hospital at the earliest.
Based on the clinical stability of the patient, he can be shifted to a higher centre or treated at the same hospital.
As for the patients' Knee condition, let him continue the orthopedicians's advise.
One of the best exercises for knee problems is Swimming, Patient can try it. It really helps.
There is also eating problems with him due to excessive vomiting
. There may be an underlying psychological problem.
It is suggested of him to visit a Psychiatrist/ counsellor for his eating problems as well as to prevent further episodes of toilet acid consumption.
Hope I have answered your query.
Wishing you a good health