I second--- that ER STAT!!! also parasites may be present have them check him carefully so nothing is missed
You never want to take any chances so if this was my son, I would take him to the ER.
If it is loose motions and bloody stolls a bacillary dysentry(Most likely). If it is frank blood with pain- to rule out intususseption of intestine. Contact ER pronto if the second one is suspected. Consult doc.