There was only one reason I did not go 150+ mph on my bikes when I was a kid.  It was because I didn't have a bike that would run 150+ mph!!!
Requardless how many times you tell him to be careful he will find out how fast that bike is.  If it can go 150 you can bet the farm he will too!!!
i  saw  129 mph  on  my bike  at 19 years  old
drove  me crazy  because  i wanted to break  130
almost hit  a cow  at  110 mph
the truck  that hit  the cow said he  was at 130 when he hit the cow...he was  ok  but truck  and cow were  lost
a few years  ago  i was on  my  1100 honda magna  [over powered  too]
new years eve..  i was passing a slow car on a bridge
as  2 cars  RACING  came up from  behind     side by side
i dropped down a couple gears  and hit  it  hard....jumped  over  100.....THANKS TO THE POWER
i  got  in front of the car i was passing  just  in  time........they  almos  hit  me  even  with me doing  over  100
on top  a high bridge over the intercoastal waterway
i  am convinced  the horse  power  saved  my  life  that night
i  think  that was my last  ride  at night  and  i stay out of the sun