Thank you for your query.
It is difficult to say what can be the cause of feeling of tightness around the upper abdomen and feeling of nausea
as a number of possibilities are there:
related conditions like hepatitis
2. Gall bladder
3. Gastric conditions
I suggest that you get him examined by a doctor who may order an ultrasound
of the upper abdomen and get the stool routine and occult blood examined as bare minimum investigations.
I hope I have answered your query to your satisfaction.
Wishing your son all the best.