I can't tell you how frustrating it is for me to loose weight and making sure Nathan gets what he needs. That is a hard thing to balance.
Nathan was diagnosed with CFRD at around your sons age. Just a piece of advice from our experience, you cannot count on the glucose test to be accurate enough to rule out the beginnings of CFRD. It was a long fight but I finally got them to give me a meter so that I could do random checks at home and that is how we got the diagnosis. Getting the diagnosis did help with weight gain quite a bit. If you go that route, anything above 110 first thing in the morning before he eats is to high and then anything above 150 any other time of the day is to high. I hope that this is not the issue, but may explain his weight issue. Also, the other symptoms are going pee all the time and always being thirsty.
I hope that CFRD is not the problem but if it is, catching as early as possible is helpful. Look forward to hearing what results you get,
Patty