The recent changes you are referring to are in receiving a Boarding Group for "Rise of the Resistance" (RotR), which is a ride within Star Wars Galaxy's Edge (The Star Wars Themed area of Hollywood Studios).
The Park Reservation system is where you get a reservation to simply be allowed to use a ticket to enter the park, in this case Hollywood Studios. Without one of those for the day in question, he will not be allowed in the front gate.
If you have a Park Reservation for Hollywood studios, and a valid ticket linked to the My Disney Experience App (definitely get this), then at 7AM guests will be able to join a lottery by which they are assigned a 'Boarding Group' for RotR. A Boarding Group (BG) is like a reservation for the ride que similar to taking a number at the Deli or DMV "now servicing #37" etc. They also issue a new batch of these at 2PM each day. If you are flying that morning, it is unlikely you will be in spot at 7AM where you will be able to get a BG, so your only chance will likely be 2PM.
Even if you don't get to ride RotR, that does not mean you cannot go into Star Wars Galaxy's Edge and see everything else that it has to offer. The only other ride in that area is Smuggler's Run where you fly the Millennium Falcon, but there are shops, restaurants, and other cool experiences. You don't need a BG for any other ride in the whole park other than RotR.
The parks are ~25-30 minute drive from the airport (MCO) once you are actually in an Uber/Lyft. If you are 'wheels down' at 8:30AM I can't imagine you would get off the plane, collect yourself, and actually get into an Uber before 9AM. That puts you at Hollywood Studios at say 9:30 which is 30 minutes prior to official park opening. They've probably already begun allowing people in the park at this point, and they have definitely allowed cars into the lot.
I don't know how late the last flight out back to ATL is, but you want to be generous with the time you allot to returning to MCO. If you stay till park close, there will be competition for available cars and some exit traffic. Add that to the 30 minute drive and give yourself time to check into your flight and get through security.
Staying the night before or after is always safer, but obviously more expensive. Depending on the time of year weather delays may play a role in your flights.
That said It would be a pretty fun marathon trip!