Disability Allowance for Asperger's

Hi, a few questions here. My brother (30) was diagnosed with Asperger's syndrome a few years ago. I'm wondering what his chances of getting on disability allowance are? What supporting documentation is appropriate to send with the application? He doesn't see someone regularly - but has occasional appointments with a psychiatrist - I'm not aware of the details of this or if it has to do with Asperger's or only anxiety (which he also suffers from quite badly). He's not the most forthcoming/revealing person when dealing with professionals (he doesn't trust them) so I'm not sure on what level they are aware of how bad his situation is. He has an incredibly difficult time coping with many things. I'm not sure how to proceed as I don't want the claim to be rejected based on not enough 'evidence' etc?

Would letters from family members help or is that not done?

Thanks in advance.