Hello
As you mentioned that your father is 73 years and having fever ( not low grade but high 103 degree),
muscle weakness , sweating pain , all these symptoms may be due to these possibilities , these include:
1 Tuberculosis , as you mentioned that chest X ray is clear but fever still present with night sweating so may be due to some
extra pulmonary tuberculosis. Get in blood for tuberculin test.
2
Typhoid fever ,for confirmation
Widal test and serology test are advised.
3 Jaundice , such type of symptoms are an indication of jaundice . Get in blood for HBASG for A , B C E .
However, the age is 73 years and fever is since long duration so may be
chronic renal failure , carcinoma of prostate or intestine .
In my take one more opinion for diagnostic purpose , so consult another physician because physical examination is important in such cases.
Good luck.