this would be much better if you take an ENT (ear nose and throat) specialist opinion in this case.
this can be a case of bacterial sinusitis
or soft palate abscess
as your doctor is suspecting.
but there is also a suspicion of some nasopharyngeal mass coming down from the roof of the mouth may be a fungal ball, that will need urgent assessment.
this also seems to be quiet a possibility as he has history of thrush
twice and is on steroids
and has takken multiplle courses of antibiotics.
you can continue the antibiotics prescribed along with any other drug prescribed but you should consult ENT surgeon urgently. your child might need an imaging for diagnosis.
hope you benefit from my answer. and wish your child a rapid recovery