Is this still running? I don't see ten steem given out yet. I'll answer anyway because I have a good one.

One of the first things you'd have to explain to him is how to use our bathrooms, especially public ones.

First he'd be pretty confused as to why there were stalls. In ancient Rome public toilets were arranged with several seats in a single room. Going to the bathroom was a social activity. So right off the bat you'd have to tell our Roman time traveler to close the stall and keep conversation to a minimum.

Once in that stall, the Roman would ask where the sponge is. Romans didn't wipe with paper, they used a sponge on a stick. There'd be a trough of running water to wet the sponge and rinse it and a bucket to put it in at the end. So you'd need to explain about toilet paper. If he doesn't like it, assure him that he'll get way fewer anal parasites this way.

Last of all, you'd have to tell him to flush.