Help restoring iPhone 4S please....

Hi there.

So sorry if this has been asked a thousand times before, I've tried to search the exact issue but could find no results.

My 12 year old son has an iPhone 4s. He recently changed the password and now can't get back in. (He says he knows the password, the iPhone would suggest otherwise at this point!) Ordinarily I would restore to factory settings using iTunes. However at this point it becomes problematic......

The phone is locked using an Apple ID that I don't recognise. We can only think that one of his friends has logged into his phone before he tried to change the password. We don't know the owner of the Apple ID or the password. All of his friends are denying responsibility.

We now have a useless phone.

Does anyone have any (constructive!) suggestions how we can resolve this?

Many thanks for any help in advance.