Hi
If he is active and growing tall in height like other children then it's fine ...some children are exactly like that in that age..they look very thin and doesn't increase in weight ...but height is normal ...
They gain weight as they age...
don't stress and focus much on that...just see for visible clinical symptoms if any...
If there are no such symptoms and he is looking and feeling active then you can leave about it..
Dietary advise would be sweet potato kheer, banana milk shakes, kheer made with nuts, milk and milk products, good protein rich foods, roots and Tuber vegetables, chew foods well, take sufficient time to eat the meals, good portion of meals, regular eating habits, rest and relax well, exposure to sun are all recommend..
Thankyou
Next Steps
please consult doctor if the weight further decreases...