Another study shows no relationship between being autistic and having digestive problems and no benefit from special diets. The possibility of "leaky gut" being related to autism was first brought up in the same study that falsely implicated measles vaccine, which is strike one.
I guess one can understand why people not steeped in science and confronted with an autistic kid would try weird stuff, but it is a distraction from things that might help. The likelihood of improvement from cognitive therapy makes it worth trying, and one can be hopeful that one day the brain can be influenced by, oh, gene therapy or affecting neurotransmitters in just the right place, so we need to keep up research along those lines, but I think we know enough about autism to say it is not related to diet or vaccines. You might as well take laetrile.