How he should go about reporting will be in his orders. Often, not always, when flying in, they have you report to the USO at the airport and then bus you from there. There won't be large numbers at OCC so YMMV.
Although he may swear in before OCS, he'll be sworn in again upon gradation at his commissioning ceremony. That ceremony takes place at the Marine Corps Museum (or it has every time I've seen it) just off base at Quantico and a stones throw from Devil Doc's home. That's the one you want to be at if at all possible. Get there early as parking is at a premium. I've never seen that lot so full as on this day.
Please pass my congratulations along to your son. Tell him to keep working on that physical fitness! It never ends.