Most likely this is a case of rectal incontinence.
He is advised a course of Norfloxacin 200mg B.D. for 1 week. If there is any infective pathology then he will recover .
If he does not recover then a barium enema is advised to rule our dilated colon pathology.
If the barium is normal then a surgical opinion is required because he may be having a loose sphincter which needs surgical repair.