Thank you for your query on Healthcare Magic.
What is the age of your son?
And what is his present weight?
Recent weigh gain may be due to genetic, endocrinal like hypothyroidism
, or due to junk food.
Hypothyroidism has to be ruled out with thyroid profile test after physical examination by his doctor.
Therefore take him to the pediatrician
for physical examination and investigations.
Hope I have answered your query.