What can I do to compel him to change the title/return the tag?

After my husband passed, I found that he had a vehicle registered in his name but never had a possession of it. It's found that a friend of my husband's cousin owns it. I have no idea what the deal was between them but I told him (1) I will cancel the insurance on his car, (2) he needs to register it in his own name. He said okay and I did remove it from my husband's insurance. Then he asked me to sign a paper saying I give him full authorization of the vehicle and allow him to transfer title, which I did sign. I keep asking him to return the tag to me but he's never done so. I also asked my husband's cousin to get it back for me but he said the guy didn't show up.