OK, well, I just asked my husband - who happens to work for Northwestern University’s Admissions Department (no, sorry, he can’t help anyone get in) and he says that it’s not without hope. A lower-than-the-school’s average ACT score is not a dealbreaker for Northwestern. As **silenus **says, other factors…well, factor in.
Northwestern, like many schools, uses a point system - applicants are evaluated in several different areas. A lack in one area can be made up for by a strength in another. The most important things for NU are the entrance essay and strong involvement in activities - ideally, a few substantial (in terms of time and dedication) activities that the student has been involved in for a decent amount of time. Flitting around doing chess club, AV and the yearbook for three months each is not what they want to see. But it sounds like he’s got some good stuff going for him there.
After those areas, GPA bears a great deal of weight, unsurprisingly. So if he’s got some good things going on elsewhere, then his ACT score won’t hold him back from admissions. Just don’t let him half-ass the essay. My husband insists that that is the single most important and influential part of Northwestern’s application.
As for the “is Northwestern worth it?” question: that’s a hard one. They are a great school, no doubt. But Illinois’ state schools are all very good as well. It probably depends on what sort of career he hopes to pursue and how much of a “good ol’ boys” network it demands. If a lot of successful people who could help him advance in his career went to NU or will be impressed by a degree from NU, then it’s probably worth it just for the foot in the door. If not, maybe you want to save your money.