I'm going to assume average body size and that he at least has tooled around enough to feel a little comfortable.
I'd go for a 4-stroke over a 2. He'll have plenty enough power... less maintenance (knock on wood)... and the lower decibels are a nice plus as he'll have more places to ride without people complaining (and I just like the quieter ride).
Again assuming adult or near adult height, I'd get a 150, nothing smaller than a 100. A 150 is small enough to work around just about any kind of trail, but he can still catch some air if you're ditch-banging or hitting jumps in a field.
I'm partial to Hondas (mostly 'cause they're all I've ever owned!).
And shoot for a used bike. As long as it's mechanically sound, it doesn't matter if the plastics are scratched and dinged. In fact, he'll probably ride better 'cause he won't be all tensed up about scratching his shiny new plaything.
Hope that helps! There's a video on the net somewhere about buying used bikes. Can't remember the link, but you should be able to find it at dirtbikeblogger.blogspot.