Congratulations to your son! DePauw is a wonderful school even with the small size and high greek concentration. The good news if he isn't planning on going greek is that there is a rising number on independents each year. But if he does decide to go here I would definitely advise him to rush because it is a really good experience to meet new people.
But to respond to your questions:
1) There are so many activities to get involved with on campus - for all students, not just greeks. Fraternity parties are open to pretty much all students so if he does want to go out on a Saturday night no one there will exclude him. Otherwise, there are plenty of people who don't really go out to the frats on weekends, and those circle of friends have their own things they do. Some watch movies, some have little dinner parties - it just depends on what group of people you befriend.
2) As for intramurals, I am a greek but I know that independent teams do exist. Little 5 which is our big bike race weekend was this weekend and there was a whole guys independent team. I think as long as you become involved with Independent Council you will be fine.
That being said, there is no animosity between greeks and independents. Everyone really gets along well for the most part, and any organization he joins will help him make great friends, even if most of them end up greek.