The usual way to test for cancer in the colon is a colonoscopy. There is a home test
https://pmc.ncbi.nlm.nih.gov/articles/PMC5461150/#:~:text=Polyacrylamid%20hydrogel%20(PAHG%2C%20Bulkamid%E2%84%A2,an%20indication%20for%20female%20SUI. Both require him to see a physician, though.
Naya707, he should see a physician. It might be something that is easy to resolve early on but not so easy later. It might be impossible to convince him.