Hello. You can try some roughage foods like carrot,apple, cucumber, nuts, pomegranate and plenty of fluids for your kid. Wait for another 1-2 days and see if it makes a difference. Mostly, in kids the main reason for constipation is less fluid intake and not having potty habits on daily basis everyday morning. Bring these food and  lifestyle changes and see if it works.

Since how long he is having this? You need to add more roughage and fluids to diet