Any mental health law firm should be able to advise him. The Mental Health Lawyers Association has a geographical list (https://www.mhla.co.uk/find-a-lawyer/geographical-list/). The Law Society has a Find a Solicitor page (https://solicitors.lawsociety.org.uk/), though you need the advanced search form as “mental health” isn’t an option on the first form.
You’re right to worry about a recall that long, though maybe he agreed to remain informally or maybe his CTO was revoked. You might be interested in this topic: S3 assessment if a CTO recall lapses.