Hi, unfortunately the police are right as the pub car park is classed as a public place. Even if your son wasn't driving it but just sat in the drivers seat with the keys he can still be arrested and charged with drunk in charge. As to what will happen to him that depends on what the readings were when they took a specimem of his breath, he would typically get a ban and a fine imposed by the court.
Like anything else he will have a chance to put his case forward but as previously stated he was commiting an offence and he should of got someone who haven't been drinking to move his car.
His solicitor will know all this and if he can find a loophole he use it.
Unitl he is convicted he will still be able to drive only once he is convicted and when/if his licence is revoked will it mean he can no longer drive.
hope this helps, good luck.