Autism diets always have struck me as being a bit scammy. A desperate attempt to find something to blame for behavior issues. A magic bullet to end tantrums.
But here’s the thing. Some kids with autism actually have gastrointestinal problems. It’s a perfectly normal, but fairly rare, type of thing for any kid to have and autism definitely doesn’t exclude that.
So if you think your kid has a gastrointestinal type of problem, talk to your doctor and get some tests done. If you suspect a food allergy, talk to your doctor and get some tests done.
If they find something, you can adjust your kid’s diet accordingly. Don’t start removing major food groups like grains without talking to your doctor because when you try those kinds of diets your kid will need something to replace the nutrients, calories, etc. that are being removed or the diet can cause some pretty serious health problems.
But just based on the small percentage of kids that have these kinds of problems gluten and casein are probably not the problem with tantrums, echolalia, stimming or any other autism type of symptoms and those behaviors will probably need to be dealt with in ways other than diet.
I know people that need to go gluten/casein free (adults without autism). The good news of finding out your kid does not need a special diet is that working on fixing behavior issues seems like much less work than eating like that every day.