My Husband Has A Biological Son Who Searched Out The

My husband has a biological son who searched out the birth certificate and found him about 10 years ago. This son's biological mother told him that there is a chance he was actually born as the result from her being raped by my husbands father. The son is now 42 years old and knows of this rape/possiblity and that my husband may not be his actual father. Should we do a paternity test or will it damage him further to find he's the result of a rape? He seems to want to know.