Wow it sounds like your son is doing great and growing like a weed!! I know it is scary but your son is healthy now and if he has CF there is a lot that you can do to keep him healthy.
It is certainly possilble your son has CF. 64 is actually a positive result on most sweat test "scales" although sweat chloride can be elevated in newborns (i think that is just the first day or two though). It is not uncommon for symptoms to develop much later than two months. Some kids don't have any digestive symptoms until a year or so and lung issues may not really show up until much, much later in life since CFers are born with healthy lungs and damage occurs after infections. It is also possible your son is just a carrier if i am understanding your post correctly. I think it is great the doctor's are treating him just in case though. Are they doing genetic testing to try to find the other mutation. Are they waiting until then to make a diagnosis? Some people with positive sweat tests are diagnosed when only 1 mutation is known although i guess they probably have symptoms.