I'm looking for single line phones that ring LOUD...1 of the users is hearing impaired. He also does not want to draw additional attention to his hearing disability.
As a person who wears hearing aids I let everyone know ahead of time that I wear them and that they are not perfect. For example, I have ask my wife how her say went and in her reply I have to figure, "What does 'Fish' have to do with it?" I realize that it is important that co-workers know that I have a hearing problem and if I misinterpret something it's not because I am stupid...I didn't hear it correctly. Being in the construction trade there are more noisy environments than quiet ones.
Anyway, I assume that the volume on the handset is okay since it wasn't mentioned in the OP. That leaves internal or external device.
External ringers or strobes would do the trick and would also draw attention to the fact that he has a hearing loss.
An internal loud ringing set would also do the trick and would also draw attention to the fact that he has a hearing loss.