I've been through four-plus teenage boys in the last few years (2 adopted ages 18/20 now, 2 foster ages 19/21 now, and a few of their friends who showed up one day and didn't leave for months) and I'd agree with the people who suggested money. If he's a typical 18-year-old I can't see him wearing a cooling necktie... there's a reason they show dorky old suburban people wearing them in the commercials. ;)
Unless you know him very well, selecting a fabric he'd like would be very difficult... there aren't a lot of ultra-cool fabrics out there for 18-year-old boys who run the gamut from nerdy MIT-bound geeks to wannabe "gangstas"... and some nerdy geeks who *think* they are gangstas!
If you're opposed to cash, consider an iTunes gift card--my 19-year-old recently specifically requested one as a gift.
Of course, if you know your nephew quite well and he's always expressed an appreciation for your handiwork, then by all means, make him something. :)