I would also ask why the social worker did not ask for a solicitor. It is ultimately the defendants choice, but can certainly be asked for by the social worker or "appropriate adult". It is not there job to legally advise. There role is to make sure the junvenile (your son) is aware of his rights and will convey to him in such a way that he understands.
I assume your son was interviewed, either on tape, video or contemporaneously (written).
Do the Police mention this evidence anywhere on the interview? Get a solicitor now, it will be free. He or she will obtain a full copy of the police interview under disclosure.
Forget forensic evidence, your sons DNA or fingerprints will naturally be on the window anyway. As for witnesses, then this will be disclosed to the solicitor as well (not compulsory, but likely)
Even if "breaking in" as you believe the police were after, I fail to see how criminal damage would apply if at that time it was your sons abode.
Very odd.
All I will say, and please do not take this the wrong way, is are you sure your son is 100 percent with the truth? Mnay parents only see the sun shine out of their kids bum and fail to see any bad. Kids, especially when backed in to a corner will lie more convincingly than a politician.
However, if you are sure, there seems to be a few abuses of process from both the police and social services.
What a waste of public money.
Good luck with the boy. If what you say is right, he sounds a good kid.