In order for one person to own a life insurance policy on another person, there must exist an "insurable interest" between the owner of the policy, and the person to be insured.
An insurable interest basically means person who owns the life insurance policy stands to lose financially if the insured person were to die.
There does exist an insurable interest between spouses, siblings, parents and their children, and business partners, among others.
So, yest your child may buy a life insurance policy on his father.
However, in order for him to do so, he would usually need your permission, your signature on the application for life insurance, and you may need to answer some health questions, and/or take a physical examination to find out if you qualify for life insurance.
Also, there may be an age requirement by the life insurance company, requiring an owner of a life insurance policy (your son) to be a certain minimum age, for example, at least 18 years old to own a life insurance policy.
Here's how you can learn more about Insurable Interest and how it works.