Ok, here are the two short answers:
No, he doesn't "kind of have CF"
<b>Yes, there is a difference between mild, moderate & severe</b>
The long answers are:
With the advent of genetic testing and screening it is getting easier to find people with the more "mild" cases -- because they don't always present with all of the more classic CF symptoms.
I don't have the info in front of me at the moment - I'll try and find it for you, but there are definite criteria for labeling someone with CF as mild, moderate or severe. It has to do with many factors such as lung function, infections, hospitalization, and digestive issues.
The good news is he is mild now. That could be the case for many many years to come. Many people are quick to point out that what is mild one day can turn to severe the next and to a certain extent that can be true (not literally one day most of the time) but from what I have learned the majority of the time things gradually changes over time. The stories vary greatly from things like "I was working a lot of hours and just wasn't taking very good care of myself" to things like "I never quite recovered from one infection, continued to loose weight, then got another infection that was resistant to the antibiotics and I never quite recovered to the point I was at before" Progression can and does take years. With many people living very full lives. There are many people on this site who are in their 30's 40's and 50's. I personally know of a man who is 76 years old and still doing quite well.
The glass is half full - as our CF doc always says. Based on the limited information you have stated, I would think you have every reason to believe your son will do quite well for many years, well into his adult life. Although there are no guarantees and no crystal ball the biggest thing he has going for him is early diagnosis and not having any digestive problems -- being able to properly absorb the nutrients and gain weight will help his health tremendously.
The diagnosis of CF can hit a parent (or patient) pretty hard and I'm sorry this is happening. Try and remember many things that you read about may be out of date information and/or may never happen to your child especially if he is pancreatic sufficient. The best thing to do is find a certified CF clinic and good doctor, keep learning all you can and have a positive attitude.
I for one have no problem claiming my kids are "mild" not only do they fit the clinical criteria but I can say based on what I have read from others on this list, we just do not have the issues that many others have dealt with for years. There is a huge difference between having a 18 year old who has only had oral and inhaled antibiotic and the 2 year old who has had multiple stays in the hospital.
You can read my blog for more details about my kids and their health if you are interested.
I wish you well - if I can help in any other way, please let me know.