Sad news that. I've had a 20% hearing loss since birth myself so can partly imagine how he is feeling. Hopefully it is only limited to one ear and that he can adapt to the change. I hadn't realised until I was 15 or so that I lip readed what people were saying to help me hear / understand what they were saying.

Doesn't always work though as I can sometimes get the wrong end of the stick and go off on a tangent with like sounding words

Most workplaces will help out with disability adaptations such as phones with adjustable volume / volume ampifier to assist him if need be.

I know you will but give him as much help as you can. Hope things work out for him