I also agree with the the first Two comments. He should go see a doctor and have a colonoscopy. It sounds like a horible test but theirs actually nothing to it. It is the best tool doctors have in diagnosing Inflammatory bowel disease and your sedated for the entire procedure which ususally takes about 30 minutes.
I myself am 24 years old and was diagnosed with Ulcerative colitis when I was 11. I had the same symptoms you described your son as having. Knowing how painful and diffucult UC can be to live with, I reccommend your son get this done as soon as possible.
i agree the sooner he is tested the better, my brother has UC and when he finally went to the docs and got diagnosed his area of intestine effected by UC was over 2 feet !!! but now after treatment its less than 6" and he rarely gets any attacks because of his medication. Good luck!!!
he should go to the doc and get examined. pain and bleeding are not something to be ignored. a family history of IBD does increase the chances that he has UC. but it can be successfully treated and starting right away will help. good luck. I know it'll probably be hard to get him to go to the doc.