It would appear you have a good case. But in addition have you contacted the police to lodge possible identity theft, they will probably not be interested in doing anything but best to have these things recorded against future issues. The individual may be using your son's name for other things and that could in the end impact on his credit score amongst other things.
Also the above point about the bailiffs is very valid. Our neighbours when we lived in West London legitimately purchased their bungalow but had over 4 years of bailiffs turning up, demanding entry even when they were not who the documents were issued for. Sometimes these guys were very aggressive and seemed not too concerned someone else was living there or in upsetting the neighbours either.
Also assuming it is all sorted well in the end, and I am unsure of the legal process (may need a solicitor), but you must ensure the record is totally removed, not just that in this case the case is not proceeding.