My son is also 15 and the ranges we were given are the same as the ones Toni quotes.  I am surprised that your son take his med only twice a day, the dose of hydro may not be lasting the full day. Has he had a 24 hour profile done?  It may be that he needs an afternoon dose?  What time was the blood test taken?  Sorry to hear you are having problems, please let us know how you get on.  Wishing you all the best

Mary