I live around the results of a long term a brain injury, alongside other disability. Some has improved over time, some hasn't.You wouldn't know by looking at me. Entirely separately one of my Dc's has severe acquired brain damage. You would know looking at them. Yet some of the issues affect both of us at the same level.
Further to PhilippaGeorgiou excellent post: I'd write up how exactly it's affecting him, and what it has changed, ie the affects on him and those around him.
You may find it helpful to break this down into headings of cognitive, physical, emotional, and behavioral effects.It's a common method of explaining living with brain injuries.
If he suffered post injury amnesia for any length of time, mention it somewhere.
On the one hand it means nothing to them, on the other documented longer term PIA is often a 'rule of thumb' indicator evidence towards how severe an injury is likely to be, so it can help tie up what is being said if that makes sense.
It also may be appropriate to look at what he would do with a PIP award to both help him deal with additional expenses caused by it, and aids to executive function, therapies, etc, to try improve his situation. It helps to be ready if asked, but also helps focus you on what he needs that he isn't maybe getting.
IF he'll co-operate, it may be appropriate (once he's been through security) for him to ask to hand the call over to you to describe the reality of how he's affected, and what his needs are, especially if you're caring for him.
Don't forget to look after yourself, it's a means to an end, but preparing for and going through this stuff is additionally draining and depressing.