In the UK, it takes a trial by jury before a judge can pass a sentence that includes someone being placed on the sex offenders register. For trial to happen the police start from the premise of needing a witness willing to testify and some corroborating evidence. Hearsay (ie your therapists word) is not acceptable. The UK is going through a period of looking at historical sex abuse of minors by celebrities..... since there is only one witness to each event and no corroborating forensic evidence, they rely on multiple independent people testifying in public that similar events happened to each person showing a pattern. This is notoriously hard to do.
What is worth considering is that your fear of someone being put on sex offenders register is a false fear that was planted in you by the offenders/colluding bystanders to ensure the secret was kept. Offenders/colluders us things like "you will not be believed... it's your word against theirs", "they're an adult/respected etc etc" and " you wouldn't want to destroy his life would you.... not for something like this...." these messages are an integral part of creating the warped power dynamic that is so damaging to people who've been abused. The abused person struggles to escape the deeply implanted false truths that are given by the abuser and not refuted by other healthier more objective people in the abused persons life.
The question you ask is jurisdiction specific and can only be answered accurately by a lawyer or someone experienced in the legal framework around this type of situation in your country. If it is something that concerns you and is blocking your growth then asking a lawyers opinion on the LAW, not the incident.... so you are not giving any identifiable info may help... in addition, lawyers are bound by legal privilege which means conversations between lawyers and clients are completely confidential... this is a crucial part of the legal system... lawyers CANNOT betray their client.. if they do so they can be barred from practicing as a lawyer.
You could also call an anonymous child abuse helpline.... the helpline operators are trained to signpost people to appropriate resources, which in your case may be a legal advice service.