Firstly, if you say your son has been beaten up by prison officers and been subjected to psychological bullying, then you need to report it to the appropriate authorities. The first step would be to make a complaint to the prison where your son is currently held and ask them to fully investigate it.
He can only be prescribed anti-psychotic drugs under the direction of a prison doctor, and therefore he must have a sufficient mental illness in order for the doctor to justify prescribing them to your son.
The title of your question says he has been "upgraded" from a Category B to a Category A prisoner, which suggests that he is a danger to the public and/or he is likely to attempt escape. This could have been under review by the Prison Service or a Judicial Comittee, or on rare occassions, by the Home Secretary. Although they are unlikely to tell you, it may be worth writing to them and asking why he has been "upgraded", although I would suspect it has something to do with an underlying mental health condition.