My husband sold his car to a Hyundai car dealer.What does he have to do?Or does the car dealer send the documents to the DMV?

You can surrender the plates at a DMV office or cancel the registration in a MyDMV account. https://dmvapp.nv.gov/DMV/Application/DMVPortal/Pages/Default.aspx If you don't do either, you will get insurance notifications when you drop the insurance. If you do, you'll have a credit for unused portion of the registration.

Dealer will do everything.

I would surrender your plates on that car to dmv..take check reciept showing you sold car