Also if he wants to play around and weave in and out of cars at 30 mph+ and he ends up as a road waffle too bad, so sad.
Eh, depends on the location. My sister rode a bike every day for about 4 years (she didn't have a car). She always rode on the sidewalk because she rode at a slower pace, the sidewalks generally didn't have any people on them, and in her neighborhood if you tried to ride in the road you'd get mowed down by vehicles. There's really some places where it makes most sense to ride on the sidewalk even if you aren't supposed to.
Regarding the general question of danger to pedestrians: good urban planning works wonders.
The next step down is electric scooters, which are around $1000-$2000. They aren't pretend bicycles, but are more street-suitable.
If you want moped speed, get a moped. You can't safely upgrade one component; you need to upgrade them all. And it's not just your safety, it's the safety of the public, and they have a right to legislate that you don't drive a dangerous vehicle on the public highway.