The assessment would likely be an educational psychologist if he was of school age and would look at reasoning, memory, cognitive, all of which can be staged and compared to age norms.
I'd guess his legal team discovered the evidence of assessments and diagnoses and are planning to go for diminished responsibility with the aim of reducing consequences as much as is possible on the strength of them. While I have no doubt he is vulnerable, and that the Greens undoubtedly have a track record of putting vulnerable and unqualified people with significant capability issues in front line positions on the strength of their inclusive credentials, and that this should be fully investigated - he is going to struggle to argue poor bewildered and unwell child who is not responsible for their actions when he has been able to do things such as organise Pride.
Main thought really is who the actual fuck are these appalling, ethics-devoid, ideology-manic, self serving bloody political parties in the process of stuffing into council and MP seats? How many are zealots, there to do nothing but push their own personal agenda and make the world work for them personally and sod everyone else - in this case which would have included several very questionable agenda? So many of these modern days people - of all parties - seem absolutely incapable of any concept of impartiality, duty of care, sense of service, capacity to represent anyone or anything but themselves, or think about anything but their own next big opportunity. And as in this case, demonstrating basic, severe capacity issues. The Reform councils with the inexperienced but wanted to people representing them seem to be achieving very little but a lot of in-person social media style bickering, one upmanship and goal scoring. I suppose this kind of candidate would be good at that, but God help the area and people they were supposed to be serving.
It's becoming a job where a) wanting to do it should immediately disqualify you, and b) significant capacity testing should be a part of the selection process.