(2-part post):
I've got a feeling of
deja vu here. I'm sure that you've already asked this question.
The police will refer to matter to the Crown Prosecution Service. The CPS will decide which motoring offence to charge your son with.
If the CPS take the same view that the police appear to have done, (i.e. that the appropriate charge is one of 'dangerous driving'), your stepson is in serious trouble. Dangerous driving can be dealt with by a magistrates' court or it can be referred to the Crown Court. In a magistrates' court, the maximum penalty is a �5000 fine and/or 6 months in prison. Before the Crown Court, the maximum penalty is an unlimited fine and/or 2 years imprisonment.
Unless there are 'special reasons' (which I'm sure won't apply in your stepson's case) the court is obliged to impose a
minimum one year driving ban and order that the driver must take an
extended retest before regaining their licence.
http://www.cps.gov.uk/legal/section9/chapter_b .html#33