Probably he's not keeping well physically and/or emotionally. He could be stressed out due to some perceived stress in his environment. I believe that he's young child, so he would need a lot of positive attention, love and understanding from his family.
Next Steps
Please take him for a proper doctor's evaluation.
Health Tips
Give him time, play with him, talk to him about his friends, tell him stories, take him out to the park, give him opportunity to get physical activity.