Mark, techniclaly he can't "state" register his boat. He's a stateless person living in that great unicorn ranch, Washington
D.C. Not a state. Although I'm sure DC has some provision for vehicle registration, it ain't a state.
Similarly, if DC has a use tax that may apply to him, even if the boat doesn't come into DC waters for years down the line. It pays to check with the DC DMV folks and to read the actual laws they cite, not take anyone's word on the phone
Except, perhaps, the USCG Vessel Documentation Center. Great folks on the phone
, and they tend to know their stuff. I'd suggest calling them immediately to find out if the lapsed documentation will create any issues, or if there is a specific grace period that might apply. (And if so, act within the period.)
These days...most states have this stuff on the web, so researching & confirming it can be a PITA but at least you don't have to go out chase down the printed versions to be sure.