There is no official patron saint for autism; however as has already been pointed out in this thread St Dymphna is an appropriate saint to pray to for an autistic child.
Four of my five children were born on the autism spectrum and my late brother was also severely autistic.
This is an Australian record and I would not wish it on anyone.
I started this novena several years ago:
Novena to St Philomena the Wonder Worker for the healing of autism
Each person (who has autism or an autism related condition) whose name is sent to be included in this novena will be prayed for regularly by a priest of Lourdes, France at the grotto. Only the first name of the person is needed for the purpose of this novena. Everyone is invited to send names: parents, grandparents, teachers, doctors, aunties, therapists etc.
The novena is held twice every month from: the 1st to the 9th of the Month and from the 10th to the 18th of the month.
(My oldest child received a big healing from autism through prayer).
Other related information on my website:
Autism topics: general and educational
A Favour granted by St Joseph in relation to caring for my oldest child who had severe autism