15 year old son with severe anxiety not attending school, possible autism / ADHD - help to know where to start for tests

Is your brother close? It sounds like he could first use a male mentor. beyond biological and social, there are differences between males and females to where they can use someone older as a role model within their own gender. Sometimes it even helps an adult assuming that role become responsible, like the 'art' of parenting. No one really goes in fully ready.

For kids who aren't quite adults but also aren't children, they just won't know enough. They might not see any successful adults who are autistic who they might feel they relate with. As humans we tend to understand ourselves as reflected in another. 

Technically, one autistic difference with how we use language and communicate can be beneficial, as we're not hypnotised like normal society into corporate adverts. The analytical brain also cannot reason well if it is too concerned about semiotics - meaning and symbolism, or associations and inclusion. It's important for him to be able to discover where he excels and allow him space to do and become without stress. 

My son loves skateboarding, it is a together but separate sport. Autistic individuals tend to work well in these forms of sport which I only have myself to compete against. We don't have the ability to pick up unspoken group social cues, which one needs for something like football. We want to feel accepted. When in a social setting with designated rules, order, specifics to talk about, we can feel 'normal'. But when thrown into NeuroTypical social groups with nebulous constructs of communication, fluctuating rules and unspoken social expectations, it's not only stressful and overwhelming but can create issues of safety (especially with boys - dominance and hierarchy can be taken out on each other physically and aggressively). 

If he can focus on learning by himself rather than at school, I would encourage that. And encourage wherever he feels confident with socialising. Most autistics enter society completely traumatised by being forced into an environment they cannot learn in, and social constructs they will never win in, this would be part of what your brother probably went though. The goal of schools shouldn't be social but intellectual, and unfortunately this just isn't the case. My son couldn't learn a thing for fear of the other lads. His dyslexia went unnoticed because he kept his head down and tried to say safe. That's not a good education as far as I'm concerned and he was mostly with his father at the time so that was a struggle. At the time I knew nothing of my own autism. His father has since passed and once out of school, he started to excel far more than he did in. 

Ideally, follow wherever he is feeling a small 'win'. Socially it seems he has an outlet. Wherever he learns the best, allow this, so long as he's growing intellectually. I believe we mature slower than N-Typical society. But we can mature more fundamentally. It's better to focus on giving him the best set up for life, so by 30, he's a desired specialist. 

As for the father, boys should outgrow their fathers as girls their mothers. There is something unique to this that is similar to taking a baton or the memories from a previous generation and carrying on. But it is just as uniquely important boys have a healthy relationship with their mothers and girls their fathers. It doesn't make it less easy but the damage psychologically is much less severe at odds with our same-sex parent.

Thank you for your reply. you gave me some answers to questions I had. Fingers crossed he will find ways of coping and hopefully I won't muck them up with my lack of understanding.

Thank you for your reply. you gave me some answers to questions I had. Fingers crossed he will find ways of coping and hopefully I won't muck them up with my lack of understanding.