Here in New Hampshire, there's a McDonald's that's offering $20-$22 per hour for overnight shifts, full-time. Walmart is at $10/hr, full-time, with benefits--which for some people would be more attractive than your son's $15-$20 because a) Walmart would be permanent, not temporary, and b) a perception of better working conditions--outdoors vs. indoors; heat vs. AC, bugs vs. no bugs.
I ran a quick check for landscaping jobs on our local Craigslist, and some construction and general labor jobs as well, as those would probably attract the same applicant pool. There are one or two jobs with wages in the $12-$15 range, but most are in the $15-$20 range, with a few going up to $22. I have no idea how the cost of living varies from seacoast New England to Chicago, however. But it does seem as if the wage range your son has isn't too out of line with reality.