We need evaluate your child to first rule out any organic cause for his problem. It needs to start with an ultrasound and depending on his results we need to proceed further. If no definite cause is found then he needs psychological conditioning.
Next Steps
You should consult a Pediatric urologist, preferably with an ultrasound KUB.
Health Tips
There is no need to worry. Whatever the cause be can be sorted out. However assessment of the child is necessary to avoid missing a condition which may affect his kidneys in the long run.