Secretion of mucus that may stain underpants can result from rectal prolapse, condition when a small amount of intestinal lining is pushed out from the anal opening.
This, usually, happens because of straining.
Small round, and hard stools are sign of constipation.
Streaks of blood, are probably, due to the small tears of anal lining (fissures) caused by hard stool stretching the sphincter.
Constipation with mucus, could be symptoms of irritable bowel syndrome.
Mucus can also indicate an inflammation disease of colon.

It will be good to talk with his physician!

Best wishes!
Marija