I live in California so I do not know other states are different or not. I assume that you live in California. If the buyer wants document it he will need a CG Bill of Sale from you. If the boat has not been CG documented before he will need some builders paperwork, hopefully from you.
The decision to use a documentation service is up to him and at his expense.
I know some documentation services. If he do not take time to complete the documents, you can tell him about a documentation n service. A link is below.
https://uscgdocumentation.us/