Does he have the registration tag that matches the stickers on the boat from the previous owner? If so just walk into DMV and tell them you need to transfer this into my name and renew registration.
If not bring the boat numbers in with you into the DMV let them know you do not have the regisration tag from previous owner or lost it. And let them know you need to transfer into your name.
Either way DMV will let you know what’s what.
Cheers!!