I think there may be some emotional or any other cause related to mental sphere.In deep of his mind he may suppress some grief or some painful sorrow or etc.
Next Steps
Consult psychologist immediately and take proper treatment.
Counselling and psychotherapy is very much advisable in this condition.
Health Tips
Dont scold him.Behave like friend to him.Dont ever compare him with any other person like ur relatives or any guy of his class or neighbors etc.Try to increase his confidence level.