Can I prevent my 5-year-old son from testifying in a case if he was served a subpoena?

There are several privileges that exist in the legal system such as the client attorney privilege, the clergyman privilege, the doctor patient privilege and the husband-wife privilege.  There is however, no privilege that is recognized for parent and their children.  I am not sure what kind of case this is, but there are ways for the child to be examined in court in a non-threatening way that permits the court to evaluate the child's credibility and testimony.  However, you are not going to be able to completely bar your child from testifying if the child is under a subpoena.  A subpoena is a court order that requires the person to come to court to testify in a legal proceeding.