Is a young man who occasionally finds a slight trace of feces after wiping his anus considered to be suffering from سلس (fecal incontinence)? And what is required of him if he discovers this at the end of the day? And is he obligated to check for this regularly?

If the traces do not always exit, then it is not considered سلس ( سلس means perpetual or continuous discharge), and you must wait for their cessation, then perform ablution (wudu') and pray, even if you miss congregational prayer. The prayers performed before discovering the trace are valid, unless there was an indication of the trace exiting during the prayer, in which case it must be redone. It is not permissible to delay prayer until its time has expired. You are not required to verify the exit of the trace by wiping the anus unless you feel it. It is permissible to recite the Qur'an and enter the mosque if you are certain you will not defile it.