You have no legal right, as you aren't related to him - you will have to ask his wife, and if she doesn't want to tell you, she doesn't have to.
Although by law, most states the wife is the automatic beneficiary of the 401K or pension plan. He CAN'T legally, in most states, name anyone except the wife. Doesn't matter how long he's been married to her.
His insurance company isn't giving you any info, because you are not the beneficiary, or the executor or administrator of his estate, or power of attorney, or legal guardian.
Your hands are pretty much tied. Unless you pay a lawyer tens of thousands, to try to convince a judge that the EX wife should be running his estate, and not the wife (and I can't imagine you winning that), you can't do anything.
Don't forget to file for social security survivor benefits for your minor children.