Applying for ESA for my son

Briefly my son has moderate learning difficulties, mild physical disabilities and autism and is moving onto a special needs course at college, following special school. We have been told he can apply for ESA and today I did that (on his behalf as I am his appointee with the DWP)
The phone call was easy enough and they are sending some forms, but I would be grateful iof anyone can tell me what happens next. I presume he will need supporting letters from GP/Paediatrician but what else?
He is not really capable of attending any interview or assessment unsupported!

Thanks