thanks for your question
is a chronic condition and its really very annoying and i can understand your concern. But you need not to worry. Steroids usually improve the condition but there are a lot of side effects of oral steroids and which will only worsen the situation if used on chronic basis. As all test results are good we can easily assume that infection has been contained still secretions are there in sinuses which are creating problem.
Best thing which can be done in this situation is One full trial of Broad spectrum Antibiotic again like Amoxycillin
, which are very good antibiotics in nasal infection
Secondly use of nasal decongestants as Cetrizine or Fexofenadine
oral doses on sos basis.
Thirdly use of Paracetamole and Ibuprofen combination to treat inflammation for 2-3 days and then on SOS basis
Forth and most important is Steam Inhalation 4-6 times a day for about 5 days apart from regular treatment. Steam inhalation will liquify the secretion and will help inn fastened recovery.
hope this will solve the problem
If problem still persist then next modality is Culture of Sinus secretions to identify target organism.
thanks and take care of your son. :)