The college is inviting him. It’s on his list. It offers favorable tuition and potential merit.
Why in the world would you not encourage him to go? Yes, it probably will have no effect on admission or not – that’s what the Common Data Set tells you. The Common Data Set does not tell you whether the Diversity Day could have any effect on merit offers. I don’t have a strong idea about that one way or another, but I think there’s at least a decent possibility that he could help himself by impressing people at the university. (Whether he’s the kind of kid who will show well in that context is another question altogether. But if the college is using the event to identify the students they most want, I assume they will have people there who know what to look for when they are meeting teenage boys, and aren’t just going for the brightest smiles, firmest handshakes, and slickest humblebrags.)
What’s more, maybe he’ll really like this college, and be enthusiastic about it. That would be great.
“10 hour round trip drive”: If that’s five hours each way, count yourself lucky. You’re not likely to get out of college selection season without a few of those, unless your kid is really committed to being near home. Anyway, can he go on his own, without you? Is there a bus, or could he get a ride with someone?