It is completely illegal for your ex-husband to take out an insurance policy on you and your son from a previous marriage, without your consent. However, I suspect that's not what is happening here.
It is legal for your ex to continue paying premiums on an existing policy taken out on you and your son. You would have signed for this policy as the insured, and he likely would be the named owner on the policy. If your ex is receiving the bills and paying the premiums for this policy, the only way that's possible is if he is the owner. Unfortunately, once you gave him ownership rights to your life insurance, that can't be undone without his consent.
The owner of a life insurance policy not only is the one who pays the premiums but also is the only one who could change the beneficiaries. I suspect that he must be the primary beneficiary, if he's willing to keep paying the premiums.
I wish I had better news for you. I always recommend that an insured person and the policy owner be one and the same, in most cases. (You know, you could always get even with him by taking such good care of yourself that you outlive him, and he ends up paying all those premiums for naught!)
A word to the wise for anyone going through divorce: Make sure that, as part of the divorce settlement, the ownership of life insurance policies is resolved ahead of time.