It's always a mixed blessing when you decode your teenagers desires. Good to know, bad to endure!
When my oldest first wanted to ride, we shopped for bikes. He was also intimidated by larger displacement bikes. I had to tell him, even 350cc twin bikes with 21BHP will kill you. They go fast enough to put you in harms way. So, instead focus on the nature of the bike and the style of the bike you'd like, and let's go from there.
Of course, training, parenting, safety gear, are all requirements in our household. But if Logan can survive riding a bike, it should make him a better overall driver. And it might just "slow him down" a tad behind the wheel
I may be get burned in effigy for offering this: since he's just done a nut and bolt restoration, something a bit more creative may be stirring inside him. Might want to test this notion with him. And the design/decision process involved is a whole different set of challenges over stock.