Hello.
First, "extremely unlikely" does not mean NEVER. Hasn't your doctor ever heard of rare diseases??
Second, if a patient complains to a doctor about something (ANYTHING!!) then it is that doctor's obligation to do something about it. If the patient wants a test, give the patient the test. Or suggest something more age appropriate. Or something! Don't tell the patient it's "probably IBS." He might as well tell you, "I don't care about your problems, son. It's most likely all in your head."
And if the doctor refuses to give the patient what the patient is requesting, then the said patient needs to find another doctor!
That gets me so mad. I've had (and dumped!) docs like that. You wonder how they made it through med school; they're so uninformed about health. Anyway, I suggest you do some serious research (the net will probably be your most up-to-date source) about IBS and the tests that can be done to diagnose it- names of the tests, what they entail, how often they should be done, starting at what age. An informed patient is much more likely to get what he wants than someone who goes to the doctor blindly, trusting whatever the doc says.
Good luck!