About my father Health. as after jaundice he is very weak?

My father is 62 yrs old. He is a 20 years patient of Hypertension(Blood Pressure). 3 weeks back he has fever and after taking antibiotic he got yellowish and we thought it is Jaundice. Physician asked to admit him immediately in the hospital. All the tests of Hepatitis A,B,C,E are negative. Hospital doctors discharge him after 7 days of treatment with drips. during those 7 days he was having loose stool. all the stool test are also normal. after discharge of 10 days still he is very weak and after every stool he is drained out. after everyone 1 hour or so he feels like his stomach is empty and he almost vomits because of that. now problem is that he is too much weak now. please let me know what can be the problem and how to recover his weakness. as he cannot even speak because of weakness.