It may also be helpful to define what slavery is like in the gameworld. When we in the US think of slavery, I think most of us automatically assume that it's the slavery of the American south. That slavery was in fact a lot MORE brutal than other slaveries that occurred in history, where in many cultures being a slave was a legal status and there were laws and customs on the proper treatment of slaves, how a person becomes a slave, etc. Not that it's ever GOOD, of course, but it's a varied and complicated thing.
I know that what I did for my homebrew was set two classifications for slave-like labor. Slavery is the Evil, cruel and brutal thing where sentient beings are sold as chattel, have no rights at all, and their masters do whatever they please to their slaves with no reprisal.
The other term I use is indentured servitude. This would be things that have laws governing them and standards are applied. Stuff like forced labor for a criminal, a person selling their service for a given length of time to repay a debt, certain instances of war captives, etc. Abuse could still occur in these situations, but in general, the people have certain rights and must be well-treated. It's the labor that's owned, not the person.
I know that historically this is not correct nomenclature. I use it because, as we have seen, slavery is a very Heavy Word, and many folks, myself included, have a hard time thinking beyond our gut reaction to it. Rather than having cultures were slavery is common but not necessarily an Evil thing and trying to express that, I use a term that's not so charged.
As far as dealing with the player, I agree with others' suggestions to casually, tactfully ask what level of maturity and heavy themes he's ok with. There are a lot of things in fantasy settings that make for very good story but not necessarily good roleplay, and every group is different. You can't know for sure what's going to offend someone, but all you can do is ask. Try not to MAKE it into a big deal, and it won't be. I'd say just pop off an e-mail briefly outlining any of the Big Things that might crop up in the campaign and ask if he's cool with it. If he is, cool. If not...that should be cool, too. I, for instance, am ok with issues of slavery in games, but if the DM has an idea for a game that deals with the total subjugation of women or a lot of sexual themes, I wouldn't find that fun, and would not want to be booted from the table because of it. To me that's like saying "I don't care that this makes you uncomfortable, if you don't like it, leave." It puts pressure on someone to supress thier uneasiness in order to play.
Wow. This is long. I look forward to rereading it in the morning and seeing if I still feel as erudite then as I do now after a late night and too much espresso.
