High blood pressure can be a symptom of too much florinef.  The florinef helps the body retain salt, so if there is too much florinef, too much salt can be retained causing high blood pressure.  However, your son may need all the florinef he is getting to keep everything else under control.

My son had high blood pressure for about the first 2 years of his life.  It caused a lot of stress for my pediatrician, but my son's endo thought nothing of it.  Reducing the florinef caused him to lose too much salt and we had a tough time straightening him out.  His blood pressure is normal now (he is 2 1/2 and takes .15 mg of florinef/day). 

My understanding is that as kids get older their kidneys function better and therefore they often need less florinef then they did when they were younger, unlike the cortef which has to be increased as the child grows.

I don't know if this is what is going with your son, but might be something to ask your doctor about.  Do you know his plasma renin level?