If renin levels were high, your son should propbably have had an increase in florinef not a decrease!  Also if his other levels were suppressed, then why on earth has his hydrocortisone been increased?   Are you sure you haven’t got the results round the wrong way, as (low renin and undersuppression?) as then the change in meds would make sense.  

It is actually quite normal for florinef doses to be lowered as a child grows and their hydrocortisone dose is increased, as there are mineralocorticoid properties in hydrocortisone, so your son will be getting more that way.