how to explain to a 4 year old their brother has Autism?

Can anyone recomend a book to help my four year old son understand his big brither has Autism.

Also any advice on applying for a blue badge as have already been turned down as my Son's condition is a mental one not Physical they did not undersatnd that his Autism creates a physical disability and because its not all the time only when he is one of those moods will not give us one.

Thanks