who is rain man?

when i first heard the title of this motion picture--rain man, i thought it to be a man who sufffered something terrible and cried in the heavy rain. In fact,rain man is the homophonic word of Raymond.The reason for that is that when both of them were young,Raymond can't pronounce his name clearly, so in the memory of Charlie, rain man is a imaginary figure.He didn't realise that rain man is definitely his blood brother until he encountered Ray. There is no story without coincidences .People now describe the man who is autistic as rain man.