Unless you can prove that the car is needed for the child's motabilty you will not get a car. Anyway first you have to prove you qualify the motability allowance (Which is the Higher Rate of DLA) and then you can ask for a car. The car is supplied and they keep your allowance to pay for it on a leased deal. If he can walk, forget it.
I reckon you'll be lucky to get the lower rate attendance allowance. Wait until you see the forms for that application

and the decision will take weeks if not months. They say no to 8 out of every 10 that apply on the first application. Best get the CAB or your social worker to fill it out and you have a better chance of getting it but remember the money is only given to help the child .... or to help you to pay for extras that the child needs because of the disability.