It seem form your problem that your son has telogen effluvium- a condition of hair fall
due to improper nutrition in this age group. It is a common process in all the children with poor nutrition, so do not worry. Give a very good nourishing diet. Consult a good dermatologist
and have good follow up with patience and faith in your doctor. You have to give long term treatment in form of vitamins
mineral treatment, it takes multiple of months to see the result.