Usually, an insurable interest means you stand to lose financially if that person dies; however, there does exist an insurable interest between spouses, parents and their children, siblings, and business partners.
As long as he can pass the medical exam and you pay the premium to keep the policy going, you do have an insurable interest in his life and can buy a life insurance policy for him. He may need to answer some health questions, and perhaps sign the application for life insurance.