Hi sukibee,
My daughter is 22 now, and gets mobility, she's also Autistic, we were told at the time, she couldn't get it until she was five, even then it was hard, you have to fight for it,, make sure you have all the medical docs etc. check the minimum age, it may vary on the childs disability, enlist the help of your local MP.
Has he been seen by a doctor, to judge his walking ability, awareness or lack of awareness of danger?, these doctors are usually ones reccomended by the mobility people, we had to take our daughter to a hospital for that, although sometimes they make home visits
Hope this has been of some help.
'L'