I'm familiar with the work of the adolescent forensic unit you refer to which is, I believe, Tier 4 CAHMS specialist and reading your account has left me deeply concerned not only for the welfare of the boys, including your son, who were able to abscond but also for the welfare of the general public during the time they were at large.
I would suggest you seek explanation as to how this breach of security came to occur - was it due to inadequate staffing levels, lack of proper attention and/or supervision, inexperienced/agency staff etc, and what measures have been put in place to ensure that there will be no repeat?
I would also suggest you ask for an account of what time the your son's absence was first noted together with the times that subsequent action was taken, i.e alerting police. searching the buildings/grounds, notifying you that he was AWOL, etc.
In short, it seems to me that you are entitled to receive a full account of what took place on Sunday insofar as it relates to the care and welfare of your son on that date, and of his subsequent behaviour and demeanour since the incident.
In addition, I suggest that you ask for an account of how many such incidents have occured during the past year - if the hospital appears reluctant to supply this data, I would advise you to make a request under the Freedom of Information Act 2000.
It's also of concern to me that, some 48 hours after he absconded, you have not been allowed to speak to your son as it seems entirely reasonable for you to expect to be reassured directly from him that he is as well as can be expected after such an incident, albeit that such reassurance may of necessity take the form of a very brief telephone conversation.
The bottom line is that although he will inevitably be subject to sanctions following this incident, your son has been placed in this unit for the protection of himself and others and for treatment rather than for punishment although, of course, it may not seem to him that this is the case.
No doubt your own years of experience with your son have made you sympathetic to the enormity of the task faced by those who treat and care for young people who are conflicted by serious mental health issues.
Neverthless, last Sunday's incident is a serious breach of the duty of care that should confidently be expected from such a unit and I would further suggest that you appraise your son's solicitor of this incident at your earliest opportunity as it may be that s/he is better placed to obtain answers to questions which may be more numerous that the ones I have posed here.