My daughter apparently does not have a physical disability based on their criteria of a "physical disability". My child is on a spectrum that sure involves some limitations that affect her physically .
But can your child move independently? ASD doesn't necessarily impact mobility. A physical disability necessitates accessible aids (ramp, porch lift), orthotics, mobility aids (walker, wheelchair), etc. That's why children with a primary diagnosis of a developmental disability such as ASD don't qualify.
However, if your child is between 3-18 years of age and not toilet trained, you may apply for Easter Seals' Incontinence Supplies Grant Program to help pay for diapers/pullups. This is an independent program.