I am so glad that your son is getting his care from a good center and that you like his doctor. That is so important!
I hope you are able to find out his mutations when you have his next clinic appt. I don't really like it when drs say 'mild' either. In addition to what Liza posted above, they may have told you his case was mild due to what mutations he has; some are, at times, associated with milder disease. However, there are differences in disease presentation even amongst those with the same mutations so making predictions can be very tricky.
My daughter was also dx'ed at age 7, a year ago. I understand your feelings of helplessness and confusion. We, too, thought it was 'just' other issues- never once knowing CF was even a possibility until a few days before her diagnosis. It will take some time for the newness of it all to sink in, it is a big shock to get a dx like this. This site is such a good place to get support and I am so glad I found it. The social worker at your CF center might also be an excellent source of support for you (ours is wonderful!)