Oral candidal infection is not likely to interfere with BERA test as it bypasses the possible interfering outer ear structures and measures directly the status of inner ear
connections above to brain.
If you feel that your child can hear without hearing aid
please get his hearing checked by paediatric audiometry
or if it is not conclusive BERA test itself. This is likely to clear your doubt. After this if it is sure that he can hear well without aids then no need to use it.
Even with this if it is not explained show hm to a good ENT specialist
in your area who can solve the problem.