Autism is a complex disorder and every person who is affected by it is different and has different needs.
It all boils down to what you write on the form.
We were unsuccessful on our first two applications for my DS, both of which went to tribunal stage. We lost the first tribunal, but won the second.
If you think your grandson's needs warrant more than a low award, you should think about appealing.
I know people who get no award, low, middle and high rate of DLA for their autistic children.