Hello,
Thank you for posting your query at DoctorSpring.com
I think your son is suffering from Fissure in ano. In that case there will be severe pain while passing stool .
Sometimes, blood will also appear in the stool.
Movicol is a good medicine for constipation.
As there is no response, he should be taken to a Surgeon for digital evacuation of the stool. The stool might have become hard and impacted. It should be removed by fingers only.
He should take plenty of liquids, high fiber diet, fresh fruits and vegetables.
Hope I answered your query.
If you have any questions, I will clarify further.
Wish you good health.
Regards