Your son will be entitled to free and independent legal advice - that offer is given in the police station (which you turned down) and right through to court. Our force give a list of the local companies that do criminal law when people leave custody - were you given anything like that? If not, try ringing to see if they have one, or can suggest some firms (don't worry, the solicitors are independent of the police, I am simply suggesting this as it tends to the same set of firms that practice criminal law in an area).
If you want one for him for court, get it sorted beforehand, as the court process will not be delayed in order for you to arrange legal advice.
I presume he will be pleading guilty? That will go in his favour, as well as the fact he has no other convictions and is not known to the police. A solicitor may be useful to plead his case, and to try and persuade the magistrates to give him a lesser sentence. In all honestly, for a first offence, I think it unlikely (note I say unlikely, not impossible) that he would be sentenced to prison (young offenders) as there are so few spaces available they try to avoid sending them there unless they really have to.
Hope that helps a bit. I'm a police officer, and its good to read that you had a positive experience with the officers you dealt with, as so often you read posts of people slating them. If it helps, my ex-husband did exactly the same thing when he was 16, except for it was his mother's bosses car which they had at their house for some reason. He was extremely fortunate to 'get away' with a caution rather than being charged.
Good luck with everything.