I'd call the DMV to be sure, but what I would do is...
Just drive it with no plates, but keep a copy of the signed off title with you in the glove box. Also make sure that you have the insurance binder in your name. If you really want to do things the right way, sign a written reciept with his name and address, your name and address, the VIN, date of sale and sale amount. That a reasonable cop will understand what'* going on. Just have everything consistent and as legal as possible. You'll be fine.