Single sided deafness and employment

My son developed and ear infection leading to Single sided deafness in his honours year. He still battled thru the worry and graduated. This last year has seen him like many youngsters struggle to get that first decent full time job. All thru this he has preferred to not use hearing aids and to live without disclosing to others who don't know. He says he just adjusts to the situation. 

He has recently got his first job but at no time has he revealed to his employer he has SSD. He thinks he can hear so it is not there business

If the employer asks him to do a medical what are his and the employers rights with regard to disclosure. Could the employer see this as hiding something and hence terminate his contract

I just don't know enough but very concerned as I think it would be a devastating blow if it goes pear shape

Could someone advise on this