I think he sounds like a good candidate for a Utilikilt. They definitely count as "anti-slave-to-fashion" attire, they look great with boots, there's few questions about tartan (although I do get asked about my clan sometimes), they have pockets, and quite often people call my Workman Utilikilt a "Carhartt (tough work-wear) kilt" because of the fabric used, so you've got that "construction worker tough" image going for it.

If he ever indicates a concern about old-fashioned vs. new, you might take a look at them.

http://www.utilikilts.com/

Be warned though, no matter what style he wears, there will be certain questions constantly asked of him, by people that normally wouldn't ask such things. ;)