Loss off appetite , Dry. how to deal.

My son is 4 years old. He is not eating food properly regularly for last 1.5 months. He eats all things except food.  He used to eat 2 chapatis two times a day and milk 2 glasses a day and between snacks nuts and more. But from last 1-2 months, his diet has been reduced. We have to feed him forcefully after many temptation. Still he eats half of his diet. But other things he is eating properly. Like nuts , snacks milk , grams . What is the reason behind it. Also he go for potty 5 days in a week  from starting . Hard stick type. That cause blockage of pot. What syrup should I use. We are only giving him zincovit syrup for about a year daily . He has also nose allergy. He keeps picking his left nostril most of time. So he injured his nostril . (Papdi bani rahti hai) nostril keeps dry. How to deal.