I'm so sorry that happened to your son. *hugs* It's awful his best friend committed suicide and his best friend's daughter died of leukemia. Sending lots of love and hugs.
Constipation can cause flat stool and can cause fissures(tears in anal tissue) which can cause bleeding. You could try eating more fiber or taking Restoralax(here)/Miralax(US). You mix it with water or juice two to three times a day. It has no flavor. It helps me sometimes. My GI put me on Constella...
My CEA was below 1 while I still had my tumor in 2014. I had CEA and CA 125 tested. I had yearly CT scans and yearly colonoscopies for the first couple of years. After that it was colonoscopies every 2.5 years. I have them more often than every 3 years due to another condition.