K.C.
I was 14 when I had many of these same tests .... And I have had them repeatedly for the past 30 years, as I have Crohn's Disease and these kinds of tests, combined with a colonoscopy, are the only way to follow the progression of the disease. I DO NOT have cancer ... Of any kind. These tests are diagnostic tools for his doctor and if all other diagnostic tools have failed to provide an answer, then he must move to the "big guns", so to speak.
If you are still worried, use this risk calculator from the American Society of Radiologic Technologists ... http://www.xrayrisk.com/
But, according to this report (http://www.physics.isu.edu/radinf/risk.htm), his risk factor is exceedingly low and about the same as if he'd driven / ridden 357 miles on the highway.
All that said, relax Mom. You made the right decision armed with a need report from his doctor. Good job!