I don't think it's as bleak as some of the other posters make it seem, but reservations will be no help.
1) Go to the airport and ask. I've had decent luck with getting switched around when there was weather (and sometimes just when I wanted to) by asking nicely. I think the gates (either of them) are the best shot, assuming he isn't checking bags.
2) Delays at ATL compound as the day goes on. If he's arriving tonight and leaving in the morning he might get delayed inbound, but not as likely on the outbound.