good news and bad news, lets start with the bad because it does get better, your son if prosecuted can get a minimum of six points on his licence for having no insurance, this brings in to effect the DVLA rule which means he will have his licence revoked then he will have to re take his test all over again this is because any driver in a two year probation period who obtains six points must have a re test. Now the good news, if and it�s a big if he does get took to court, you need to get a solicitor, and plead the following mitigating circumstances that he genuinely thought that he was insured, you need to take all the documents that you have mentioned including your bank statement to show the magistrates that the money was taken out of your account on 23/03. and the policy which you got on the 25th.Your son or the solicitor can then ask the magistrates not to implement the 6 points stating that it is up to the magistrates discretion as to imposing these points, in mitigation he can also mention that he had no intention of leaving the scene without giving his details having left his details on the windscreen of the car that he hit. The other option is to pay the damage of the other vehicle; quite often the police do not want top get involved unless there is a sixty per cent chance of winning a prosecution.
I am sure the magistrates will take in to consideration that the money was taken out of your account on the 23/03/09 and that the policy SHOULD have ran from that date, I know ignorance is no excuse but in your case, I would most certainly give you the benefit of the doubt. Try to get it settled out of court it�s a hard and expensive way to learn, ask the I.P if he can get an estimate and that you are willing to pay, tell him if your son claims, it will incur penalty and it is cheaper for you to pay for any damage, but it may save your son a disqualification and re test