Portland -- Place to take group of college students
Do you have a budget and neighborhood in mind? Frankly, I'd just go to Kenny & Zukes for some awesome pastrami, though lately I've been attacking the corned beef. Lots of cool rare sodas like the Dublin, Texas, original-recipe Dr Pepper or some sarsaparilla. (There's beer and wine, too, if they're of age.)
One thing to consider is if your son and his friends tend to be bound to their neighborhood. In other words, if they attend PSU, perhaps you should use this opportunity to escape the SW Portland area.
Of course, there are so many options but off the top of my head I think a bunch of college kids would love either Mama Mia's (Italian) or Mother's Bistro (good ol'American comfort food). Both places are located downtown and have a nice urban vibe, but are decidedly casual (like most of Portland).
You could also think about the Doug Fir which is quite the hipster spot in Portland (but also just plain fun). They serve standard American fare, but do so amid a modernist eco-chic environment that screams Portland http://www.dougfirlounge.com/.