Can I buy life insurance on him without his consent, so my son will be beneficiary of the life insurance policy in case my ex-husband were to die?
In order to buy life insurance on another person you first need insurable interest in that person.
Insurable interest means you stand to lose financially if the person were to die, or you rely on the person for financial support.
Since you and your ex husband share a child, you do have an insurable interest in your ex.
However, in order to buy life insurance on another you also need his permission, signature on the application, and he may need to answer some health questions, or even take a physical exam when you apply for the life insurance on him.
Another option I believe is having life insurance being required as a part of the divorce agreement. But, since you are already divorced, this may not be an option.
You may want to speak with your divorce attorney, financial planner, or life insurance agent to get a better understanding of your options in the situation.
Or, you may just want to explain the importance of this life insurance to your ex-husband and see if he will agree to you taking out the life insurance on him.