You can write a letter to his attorney enclosing a copy of the subpoenas that you intend to have served on his family members. That will provide the adequate notice. Make sure that you hire a process server to serve the subpoenas, since they will do it properly and provide the appropriate witness fee that is required. If you were served with discovery demands asking for witness disclosure, make sure that you amend those to include the family members, if you haven't already included them in any responses that you have given. Good luck.