Can enlargement of the prostate apply pressure on the rectum/anus and cause thinning stool?

Rarely: It is unlikely that the prostate gets SO LARGE that it impinges on anal canal. In my 45 yrs of urology ,I have yet to see one.But,there is always a posibility

NO Unlikely: Enlarged prostate will not be cause , at 27 prostate will not enlarge , most likely causes are poor dietary habits , ano-rectal infections , fissure , fistula etc. See your doctor for examination , including sigmoidoscopy .Eat high fibre diet with plenty of fluid intake.