If he doesn't need easy access to his car, maybe you could park at the airport. It isn't all that expensive and not very far from where you will be staying. You could still get his car in and out and they would definitely let you park there overnight. Here's a detail of the prices from the Myrtle Beach airport.
Parking at the Airport
Myrtle Beach International Airport offers Short Term and Long Term parking options, as well as a Credit Card lot, which is the most budget-friendly option for longer stays.
The airport also offers designated handicap parking spaces for your convenience in each of the lots, as well as courtesy free parking for special needs passengers in the short term lot.
In addition, curbside porter services are available, call 843-916-9287 for more info.
Short Term/Hourly Parking
The short term lot is the closest to the main terminal and the best option for a quick pick-up or drop off. In this lot the first 20 minutes are free, with each additional 20 minutes costing $1, up to a maximum of $15.
Long Term Parking
Parking in the long-term lot costs $2 for the first four hours and then $9 for parking between 4 and 24 hours.
Credit Card Lot
This long-term parking lot is the best option for those keeping their car overnight or for multiple days. The fee is $1 per hour, up to a maximum cost of $7 per day.
Handicap Parking
Handicapped parking is available at no cost throughout any of the airport’s lots.
He might also be able to park at Myrtle Beach State Park. Here's a link to their website which mentions overnight parking about half way down the page.
Myrtle Beach Campgrounds - Myrtle Beach Pier - Myrtle Beach State Park