Hi, Contact the officer who interviewed him, request a copy of the interview tape and take this to a solicitor who deals with criminal law. Only 2% of rape cases are ever proven so it is unlikely the CPS will go to trial but always be prepared to fight the case in court. If your son is charged it is likely his DNA photograph and fingerprints will be taken and in some police forces the arrest will be registered on the PNC ( Police National Computer ). It is possible that the officer knows that and he/she is trying to stop this registration taking place to keep your sons CRB history clear. I jave experianced other people been arrested for rape and then the police bail being cancelled due to no or little evidence. The PNC is updated with ARREST ONLY but the CRB check will show the arrest and the offence. If your son were then to apply to be a POLICE OFFICER, SOLICITOR , CARE WORKER etc this could seriously reduce his chances.
hope this helps
Lawman