If you can get him to stand on the board and do a few deep squats to find a comfortable place for his feet. If you can't, 21.5" width and 12/-12 is a decent general astarting point. Going even for the front and back foot is good since some newbs have no clue if they are regular or goofy.
If you're ever at Hidden Valley and see guy in a loud jacket and pants riding a white Buck Ferton come up and say hi. I'm headed to opening day there in a few hours.
2011 Smokin Buck Ferton 155
2013 Flow NX2-SE
2013 Flow Hylite boa
Burton Stagger pants/jacket