An employee has been off sick for a few days, and has emailed today explaining he has been to the doctors and they've signed him off work for a stress related sickness due to recent family issues he has had, nothing to do with work. 

I've asked him for the doctor's note where it specifies the time off he has been recommended to have, but my question is, as I know it's a sensitive issue and it has to be kept private and confidential, am I allowed to inform his direct Line Manager? Am I allowed to let him know the real reasons, or just say stress related sickness?  The company is not big, and I'm worried everybody will end up knowing about this when it should be kept private. 

Thanks for your help in advance.