If I were you I would have this problem your son is having looked into by your family doctor ASAP, it could be something simple, like buildup of wax, but on the off chance it is not you'd want an MRI done to rule out AN (I would think) so that if hearing can be preserved it will be.
I myself have often wondered if I have a mosaic NF2 as I did have a second AN, it grew in the same side as the first and the doctor's weren't able to tell me if it was just the original one regrowing (left a cell or two behind with the first surgery) or if it was a completely new tumor.
Kathleen