A friend of mine has a similar accident before, going down a
slope too...that is why I always warn my kids when going down slope
on a bicycle. It the slope is too steep, get out of the bicylce and
push the bicycle down unless you are an experienced stunt rider...
If the slope is not too steep, don't underestimate the power of
gravity too... slow down by half-braking both front and back
wheels...
Many kids or young people have similar accidents because most learn
how to ride a bicycle probably in urban environment now or at east
coast park where the ground is flat...I learned to ride a bicycle
in a kampung long long time ago where going down a slope was
common...
Think the above advice too late for him though.....but hope others
reading this will take note...