Although the most common cause for high levels of SGPT is generally due to consumption of alcohol but since he is a student ,I am ruling this reason out.
The other apparent cause in your son could be prolonged use of pain killers which he might have taken for his persistent backache.
Other reasons could be obesity,high triglyceride
levels in blood or even diabetes
I advise you to get his lipid profile
Since there is no particular for its cure,I advise following things
Do regular exercises
Stop taking any pain killers
Take fresh fruits and green vegetables
Avoid junk foods like pizzas,burgers,noodles etc and colas.
Reduce weight and be in required range of BMI.
Get his SGPT repeated again after 3 months and if you still find it high,then consult a gastroenterologist
and get tested for hepatitis
markers and other relevant tests.