How long is he planning to stay? Would he quarantine for a period of time and then get a COVID test before seeing Mom? Or continue to wear a mask around her? Flying is probably ok right now but either choice there is a small (or maybe better said, unknown) chance he picks up an extra passenger along they way that you would not want Mom to be exposed to - have to factor that in.

I talked this through with a colleague who wants to do something similar SoCal to NYC. He landed on driving and camping cross-country followed by a two week quarantine before seeing older family. But he also has his own place in NY to be once he arrives. And he is planning to stay through at least September so he has the time. I think his wife is going too.