Too bad you can't quickly get him a copy of Crisis of Conscience to read during the CONvention. If he's 19, he should be able to sit wherever the hell he wants, he's an adult.
Good on you for giving him a place to land and getting him involved in REAL life!!
Other things he can do:
~Sight-see. Walk around the convention city a bit, keeping an eye on the time so he can be back in his seat for intermission. MAYBE bring a small radio so he can listen to one or two points from the convention (they always broadcast it for the hearing impaired) so if he's grilled he has an answer.
~I agree he should talk to whomever he feels like talking to. If your family tries to give him a hard time about it and insists he NOT talk to people, he should tell them that since he was announced as "No longer one of Jehovah's Witnesses", then that means their silly-ass rules don't apply to him anymore and he can talk to whoever the hell he wants.
~He can also bring an iPod or similar and listen to podcasts from "apostates", as well as just some good old worldly music (I did this at my last convention hehe).
~If he has a smartphone, he can always sit high in the nosebleeds and watch some porn!! LOL