Your son is probably talking about what's called a "health care power of attorney," which (as I understand it) is a legal document that includes your instructions about your medical care if you are incapacitated and unable to make decisions. They need to be worded carefully to avoid misunderstandings, so you are wise to consult your lawyer.
Documents like this are a recent innovation, so the Bible doesn't say anything directly about them. However, I don't find any reason in the Bible to oppose them, as long as they uphold the dignity of human life. In fact, I would encourage you to follow your son's suggestion. Just as a person's will outlines what they want to happen after their death, a document like this outlines what they want to happen if they are alive but unable to act. In both cases, the goal is to avoid confusion and conflict.
The Bible says that "everything should be done in a fitting and orderly way" (1 Corinthians 14:40)–a principle that applies to every area of life, including our final wishes. Abraham planned for his own death, purchasing in advance a burial place for him and his family (see Genesis 23:9). Wills are mentioned often in the Bible, and I urge everyone to have a will.
The greatest legacy you can leave your children, however, is your example of love and faith. Is your faith in Christ, and are you seeking to follow Him every day of your life?