As for your concern about your son riding too fast; install a digital speedometer that retains the recorded top speed. Tell your son not to exceed 25mph. When he gets home from a ride check the speedo to see how fast he went. If he exceeded 25 tell him he gets one more chance and then the bike is yours, not his
Serously, I know how difficult it can be dealing with a teen. I built a bike for my nephew and gave him instructions on how to take care of it. I told him if he abused the bike I would take it back. He ran it without oil, seized the engine then left the bike laying on its side in his back yard. He doesn't have a bike anymore.