I know it may not help, but, I was stopped for speeding mid 2004. Forgetting the actual speeding part for a moment, the officer that stopped me entered a statement in to court that said he had shown me the speed reading on the police pilot, that he had cautioned me and that he advised me that I would be prosecuted.
He did non of these but his signed statement says he did
My solicitor pointed out that for the court to side with me would be admitting they thought the officer was lying and there was little to no chance of that.
From my experience if the police want to stretch the truth to suit them there is very little you will be able to do.
All that said, to give a balanced argument, this is the first time in 20 years of driving / riding that I have experienced this type of behavior from the police.
Hope it all works out for your son.