Being under 21 and trying to find "conventional" lodgings in a lot of states is truly miserable. My understanding is, it depends by state though. I know Oregon it's 21 years+ to rent a room though (or that was just everyone's policy?). I usually opted for couch surfing, camping, sleeping in the car, etc. The latter two don't work well in a snow storm. AirBnB might be your best bet if he doesn't have anyone to crash with, and just explain the situation to the owner. You might luck out. Slash, check state laws and call specific hotels for their policies.
(PS, it fucking sucks that 18 is legally "adult" and yet you're generally years away from renting hotel rooms or cars. Incredibly frustrating when you don't have much family support or contact)