ADHD in UK prisons

My 46yo son has had 2 psychiatric assessments and been assessed as having ADHD, but with no treatment plan. Instead his GP prescribed antidepressants. He was on PIP on grounds of mental health prior to imprisonment. This has been clear to us, his parents, since a toddler.

He is now in prison in UK following offences fuelled by his neurodivergence comorbid with alcohol abuse.

This does not seem to have been followed up in prison and we are fearful that on release on alicence he will be non compliant with his conditions of release and end up back in prison. How can we help him? We are at our wits end, as the ability to meaningfully communicate is very difficult with prisoners, especially compounded with a disorder like ADHD.