Is it better to seat my single friend with my parents, whom he knows well, or with couples closer to his age, but whom he has never met?

The general rule for seating solo guests is to put them with their peers, even if they haven’t met before. Try to mix singles and couples together (keep couples at the same table together, though not necessarily next to one another). Ideally, group some people who know each other with some who don’t. If a single guest lands at a table where everyone else knows each other, but he or she doesn’t, it can be harder to break into conversation. But, sometimes that’s the only option—in which case, trust that your other friends will make him or her comfortable. After all, you’re friends with all of them for a reason, and they will likely all get along.