Talk to Pediatrician on Gastric Problem

My son is 4 years old. I train him for washroom since he was 2 yrs. He perfectly done his urine and pee. But now from last two months he did urine as usual but did not pass his stool as he did before. I noticed that when he is with empty stomach he will be alright but as he take some thing he go to washroom after 15 to 20 min of taking food and the stool is not so much in quantity but only stick to his trouser and when I preesurized him to sit on toilet seat he sits but did not paas any stool. He actually did little bit stool all over the day and every time he did little and did not paas stool but it is stick to his trouser. I am too much worried of his behavior. He paas good stool after 15 to 20 days. Kindly answer my question