If he will not pay for the life insurance policy, you can pay for the policy as long as there is an insurable interest for you to have life insurance on your ex-husband.
Insurable interest on your ex may include the two of you having a minor child or children together.
Or you might try to get life insurance on your ex-husband as part of an alimony agreement when you divorce. In this instance, you may be able to get him to purchase life insurance and pay for the coverage so you and your children are beneficiaries to the life insurance policy.