Hi and welcome to Healthcare Magic. Thanks for your question...
The burning sensation and black or bloody stools could be due to mucosal ulceration due to the irritant chemicals in the poison. If he is still in the hospital, then the doctors will keep monitorng him and with continued treatment and supportive care, the symptoms should subside. But if he is at home, please take him to a doctor urgently for further evaluation and management.
After he is medically stable, you should also take him to a psychiatrist
for a detailed assessment.
Wish you all the best.
- Dr. Jonas Sundarakumar