Hi, my daughter is the same way.  She was in the 25th percentile for both weight and length until she was 6 months old.  She started falling off the chart after that.  At about 9 months, she started to lose weight, then would gain a little of it back, only to start losing again.  In October, 10 months old she weighed 17 lbs, she just recently reached that weight again, and she is now 15 months.  At one point between October and January, she only weighed 15 lbs.   I believe she is still off the chart now, but it seems like she is slowly gaining weight now.  My endo told me at our last visit that several of his patients with CAH seem to go through this during the first year, and then they start to level off and get back on track.  I hope everything works out for your son!!!