My knowledge of DU is from a family member that graduated, attending various events there over the years, and one of my children was interested in attending. My knowledge of Elon is less, and is mostly visiting the area and friends that attended.
Academically they’re similar, and both schools are in growing areas with plenty of opportunities for internships/jobs.
DU is in a residential part of Denver, but it’s quickly becoming 4-5 story buildings. Especially close to the interstate. Elon is the classic college town. There are plenty of things to do around DU, but it’s not all about the college. That’s probably the biggest difference between the two schools. DU has a light rail stop, so it has easy access to the fun in Denver like the LoDo and RiNo areas.
I wouldn’t describe DU as preppy. Denver and Colorado tend to dress very casual.
Skateboarding is popular at DU. The access to climbing and snowboarding is obvious. You might want to check to be sure there are clubs/shuttles/carshares to get him to the mountains though.
Denver leans liberal, but it’s not Boulder. DU seems to be the usual college with a slight liberal bias. The area around Elon is far more conservative. Keep in mind that Denver isn’t very religious compared to NC.
DU is a fun school, but not a party school like CU Boulder.
I hope that helps!