Student finance help

Hello

Has anyone had success applying for student finance - especially the disabled student allowance? My son was diagnosed as Asperger's aged 5 and since then had no professional contact as we have just got on with life. It's clear university will bring huge changes and he is going to need help once there - he's moving approx 3.5 hours journey away. 

Student finance say they need a professional letter stating how his autism affects him day to day. 

Any advice gratefully received.