1) all that trail riding DOES help on the street!!! Not with traffic, but with handleing the conditions of the road...being able to handle all the bumps, dips, drops, dirt, rocks, etc..of the trail translates into being mindful of how the road is while riding. GOOD THING!!! LOTS of beginner riders don't pay attention to the actual road until they are given a reason to (crash).
You are SPOT on its the other drivers that worry me
2) 9% isn't a great rate. But it also isn't a bad one considering age, credit exerperience (at least from what you were saying bout wanting to 'build' credit), and type of bike he is buying. (though agree...used is a much better route for whatever bike he looks at)
You are SPOT on better he buy a cruiser or used
3) you really should know if your own kid can handle that machine. Is he an adrenaline junky? If so....maybe not the best bike.
He took my T for a ride and said don't sell it. And the T is slow
I honestly dont think it would be good for him.
4) If his reason is 'gas mileage' throw that back in his face....tell him to look at cruisers instead!!! Has he been on a cruiser? A good 800-950cc import can still be a really quick machine. BUT they don't make you want to go faster!! (We all have TB's...we know how a machine can make you wanna do that!!

) Cruisers get really good mileage...have you sit in a position that is more comfortable for longer periods of time (for most people), are larger and more visable to SOME people...(others wouldn't see a dump truck)....they are still really quick...
I told him a cruiser is easier to ride plus a ferring is so nice that it cuts wind, bugs & keeps you warmer at night.
5) and insurance is MUCH...MUCH...MUCH...lower on a cruiser. Has he looked at that?
Not sure but I need to eventually get him off my insurance & am currently at 7 vehicles with 2 boys on the policy & boy what a jump in price.. I remember 3 cars at under 1k