Most insurance policies contain a clause that the vehicle should be maintained in a roadworthy condition. However, it is unlikely that the circumstances you describe would lead to his insurance being invalidated.
The court should be able to tell your son precisely what he was convicted of, if for no other reason they would have imposed fines and costs as well as any points or disqualification. He needs to find out what the offences were and what penalties were imposed.
There is little likelihood that a warrant for his arrest would be issued unless there are outstanding fines which he has not paid and which could not be recovered by the court by any means. These motoring offences are not arrestable and magistrates would not generally issue warrants for those convicted of them.
However, beware if you live where ex-PC chompu (another AB-er) worked. Apparently in that neck of the woods magistrates issue arrest warrants for driving offences quite readily. (See
http://www.theanswerbank.co.uk/Motoring/Road-r ules/Question443490.html).
Unfortunately, annlinda, magistrates� legal advisers are not permitted to advise defendants on the merits of their case, only on court procedures. Access to the duty solicitor is now restricted only to those appearing for imprisonable offences. But your idea of going to court to swear a Statutory Declaration and to have the case reopened is sound.