What Causes Clear Penile Discharge?

My son, age 17, is worried with a discharge from his penis. It appears to be clear and a bit viscous and in a very small volume. He has thyroid cancer and is currently in remission but, anything with his body sets off alarm bells with all of us as he feels it is more cancer. I have been trying to have him keep a log of occurrence but, he does not keep good records. The only clue is he seems to notice the discharge after having a bowel movement. He is not sexually active, has no pain or sign of infection and no other symptoms. If it persists I will take him to our PCP. Never tried this service before and to allay his fears decided to give it a try.