The first step is to get him the help he needs, mental health beds do not simply get handed to patients with drug or alcohol problems; it's the largest exclusion criteria there is (head injury is another one), so be warned it's unlikely he will be given a mental health bed until he has detoxed - Mental Health beds with detox facilities are few and far between in the country, and it's a shit fight to get any bed at the moment.
Some of his behaviour could be down to the head injury (obviously post head injury), the CT scan will guide the medical staff on that - he may stay in a neuro ward under watch.
The bar to get sectioned is very, very, very fucking high (they are not "handed out to coke" addicts as some people in this thread seem to think), he will have had bloods that can show current and past usage and give an indication of levels.
Please don't get your hopes up he will be sectioned, he may be home / a place of safety, under the crisis or home intervention team - this will depend largely on whether they think he needs to detox and whether they think it can be done at home, not on his mental health (frustratingly).
Do you have the means to pay to go privately, health insurance, or company health insurance policies?
He may be referred to Early Intervention Psychosis Team or Ward - these people are smashing and support families day in and day out.
Start writing down a list of any questions you may have (I used my phone), ask for a contact point for care coordinator, ensure you are not (and delegate if you are) nearest relative (legally you will be). Be prepared he may not be sectioned, he could go voluntarily, or he may go home/parents, or detox.
To all of you having a shit fight amongst yourselves, perhaps leave this thread as a place to support and advise the OP hey?