Hello
You mentioned that your son is taking DOLO 650 mg ( 
paracetamol ) 6 hourly and fever subsides whenever he takes dolo .
Your son is also taking ceftriazone 1 gm twice in day from 15 days .
Tested Widal positive and this cefteriazone is most potent and effective drug for 
typhoid fever .
However there are so many side effects of dolo as 
liver damage , G I upset and very rare blood dyscrasias ( as after prolong use but in your son case already he took it for 15 day and this is prolong use).
However , there are so many drugs for typhoid fever , but I prescribe my patient 
ciprofloxacin 500 mg twice in day for 14 days . Alternatively 
azithromycin 1 gm once in day for 5 day.
But in my opinion please get in X RAY chest , blood for c b c , E S R ( if > 100 mm 1st hour  , consider about some chronic infectious disease including  T B ).
Also get in blood for Hepatitis A , B C  E .
In my opinion consult another physician and rule out any infectious disease.
Good luck.