It's difficult to determine if the person is eligible or not for full membership and health insurance coverage that includes prescription and Medicare supplemental plans from the United Services Automobile Association (USAA). You mentioned both the terms "son" and "grandparent" without providing additional details about their relationship and current status.
An individual can only receive full membership and access to USAA health insurance options if they are the child or grandchild of a service member who also had a full USAA membership. With grandchildren, their living grandparent and at least one parent must have membership. USAA offers membership in these instances to current and former honorably discharged members of any armed forces branch. If the retired military grandparent or the parent doesn't currently have membership, then the "son" needs to ask one or both to apply for it so that he can then also apply. If one or both of the family members are deceased, then the "son" should contact their loved one's lawyer and USAA to determine if their parent and/or grandparent had membership before passing away. To contact USAA, use the information on the USAA Contact and Support Center page. You can reach out to a representative of the United Services Automobile Association by phone, text message, Twitter Direct Message and email.
If the "son" doesn't quality for full USAA membership and the attached health insurance benefit, it's important for him to keep in mind that USAA offers many other services to the general public that he might want to check out as long as he is 18 years old or older or a parent agrees to assist him. These services include retail discounts, life insurance and financial and investment assistance. If he's interested in automobile insurance, product comparisons or bank services, he would still need a full USAA membership.