My 30 year old son says that he's seen blood in his stool for months. His poop smells really bad, not like normal feces would smell?

Possibilities. Bright red blood would come from near the outlet,usually from a tear/scratch in the anal skin,this is often on the surface of a BM. Darker blood mixed with the poop, comes from higher up.It can be from infection,polyps or gut wall bleeding, or even cancer.It is all abnormal and needs to be evaluated.It is more likely to be something easy to fix at his age.
See a doctor. He should see his doctor. There are several reasons why a 30 year old could have blood in his stool. It cold be hemorrhoids, rectal tear, or inflammatory bowel disease to name a few. This should be evaluated and not let go.