Does a father have to testify against his son if subpoenaed?

Yes, a parent can be made to testify against a child. There is no parent-child privilege, as we found out when Monica Lewinsky's mother was called as a witness before the grand jury which was investigating Monica's conduct with former President Clinton. Examples of privileges that do exist are those between husband and wife, attorney and client, doctor and patient, priest and penitent. Many people believe that there is a need for a family privilege in this country, but so far the federal courts have refused to recognize it. I'm not aware of any states that provide for such protection. And family privilege bills that have been introduced to date before Congress have not passed. Further, even if there were such a privilege, if the parent and child engaged in the criminal activity together, the privilege wouldn't apply. This is called the "crime-fraud" exception to testimonial privilege.