Thank you for your query.
I can understand your concern and a problem lasting for 3 weeks needs to be worked up. I suggest that you revisit your doctor and request for a checkup to exclude jaundice
. One can have loss of appetite and weakness as the initial symptom of jaundice. Appropriate investigaions will guide you to some conclusion.
I hope I have answered your query to your satisfaction.
Wishing your son an early recovery.
Rajiv K Khandelwal