It could also be inflammatory bowel disease for one thing, there are other possibilities here
. On your profile, I see that you're fairly young - it's very unlikely that your boyfriend would have cancer if he's close to your age. If the medical staff said that it was probably nothing, I would trust them on that. I know that in many situations, they're not supposed to lie to their patients in order to make them feel better, because it would likely make things worse in the long run if there was actually a problem.
I would say there's nothing to worry about at this point, and worrying isn't going to fix anything anyways. It's really touching that you love him so much. :) Instead of worrying about him, turn that negative energy into positive energy by doing something special for him!