There isn't an official patron saint of autism, yet. Well, except for in Ukraine, maybe. But for the most part, the patron saints of autism are still informal, and have not been certified by God.... but then, most patron saints aren't. Patronage of occupations, conditions, illnesses, and etc. are generally a matter of tradition. Sometimes the Holy See will make an official proclamation on the matter, but this is unusual. So we have a number of proposed patron saints of autism, and which you choose is up to you.
St. Alexei of Goloseev: The Ukrainian Orthodox Church has recognized St. Alexei as the patron saint of autism; St. Alexei was mute until the age of 13, and that's about all we know. Most of the information on him comes from this news article; I assume my inability to identify the saint in question is due to the irregularities of translating names across languages, although it is possible that this is simply not a well-known person outside of the Orthodox Church of Ukraine.
St. Thorlak: There is an American movement to make St. Thorlak, the patron saint of Iceland, into the patron saint of autism as well. The blog Autism Consecrated has been lobbying for St. Thorlak since 2017 based on his apparent asocial tendencies, preference for following the rules of the Catholic church, and routine recitation of holy prayers and hymns. Frankly, those traits seem pretty common among the sort of people who become saints, but I have no objection to the nomination.
St. Philomena: Saint Philomena is the patron saint of infants and youth; at some point, this started to include youth with autism. I have found references to this being treated as an established fact going back as far as 2007, but very little justification as to why she should qualify. One blog points out that she would not marry Emperor Diocletian (he eventually had her killed) and ties this to the sensitivity to touch exhibited by many on the autism spectrum. I suspect that her involvement is simply an extension of her helping children in general.
St. Dymphna: Older lists of patron saints will sometimes list Saint Dymphna as the patron saint of autism. She is the patron saint of the mentally ill, anxiety, depression, and a host of other ills, including abuse. It is easy to insert autism into that list; it is perhaps a little too politically incorrect for the average internet savvy Catholic to maintain that autism is basically just a mental illness, but at the same time it is not at all uncommon to include her in prayer for kids with severe autism.
St. Ubaldo: Saint Ubaldo, also called Ubald of Gubbio is another old-school nomination. Widely said to be the patron saint 'against autism' and/or for autism, this appears mostly to be because he was earlier roped in to being the patron saint of those with OCD, obsessions, migraines, and sick children in general. However, he was known for his patience and kindliness, so he is a great choice.
These are not the only nominees, but these are the ones that I could find multiple people claiming that they were basically already the patron saint for autism. Saint Jude (patron saint of difficult cases), Saint Nicholas, and John the Baptist were also mentioned, and there is no reason whatsoever that you should not come up with your own patron saint if needs dictate.