For constipation to reduce give him fiber rich food and fruits and avoid junk food like chocolates, chips and biscuits.
These food will have high calories and doesn't not interested to take food
Next Steps
sit with him and family will eat together
train him to eat by himself so he enjoys food as well understand the need of food.
for milk intake till he accepts milk add chocolate powder slowly try to decrease chocolate powder added to milk generally he will start accepting
Health Tips
check his weight and height monthly you will know his trend
syrup Aristozyme give it 5 ml two times a day for 15 days
deworm him with syrup zentel 400mg/10 ml 10 ml at night once and repeat every 6 months upto 5 years.