Insurance co has paid benefits to the wrong person what can be done

Legally, the ex-husband sues the insurance company for violating their contractual (an insurance policy is a contract) obligation to pay him as the beneficiary: a beneficiary of the contract, he have a legal interest in it, and therefore has standing to sue. He could also sue the son as well, for improperly taking money to which he is not entitled, and can include both (insurer and son) in the same lawsuit.
Practically, he would need to find evidence or documentation showing it would come to him. That is the policy (and his proof of his own ID; e.g. driver's license, birth certificate, etc.): the insurance benefits are paid out to whomever the policy indicates they should be paid--the divorce decree or other agreements have no effect on the insurer's obligations, which are solely as set forth in the policy (which again, is a contract) itself. In the lawsuit, he can request the policy from the insurer, which will have to search for it; but note that if neither they nor he can find any copy of the policy showing the identity of the beneficiary, he likely will have some difficulty proving his case, especially if the son claims or maintains that he was the proper beneficiary and there is nothing to contradict that.