To be honest, his insistence would be weird even if he was sure. It's surely up to you when you tell people. Especially your son. You know him best. I might be missing something, of course, but a doctor ringing you up to insist you tell your son? That seems weird. In addition to what others are saying.

Could you get a second opinion? 

Hi thanks for replying means alot when doctor phoned with blood results he asked if I had told family and my son told him I had told family but not my son as he has exams and I wasn't telling him he then said you know your going to have to have this talk with him sometime I feel so guilty if I hsve put my family and daughters thru this who had taken it badly ,how would I go about having a second opinion thanks x

Hi thanks for replying means alot when doctor phoned with blood results he asked if I had told family and my son told him I had told family but not my son as he has exams and I wasn't telling him he then said you know your going to have to have this talk with him sometime I feel so guilty if I hsve put my family and daughters thru this who had taken it badly ,how would I go about having a second opinion thanks x