There was some stuff about Secetin treatments for autism. This is normally for persons with Celiac Sprue. But some say it helps persons with autism. I think Dr Bernard Rimland (vitamin B-12 treatments) mentions secretin.
Temple Grandin mentions two types of autism: the regressive/epileptic type and the Kanner/Asperger type. Some treatments work for one type but maybe not as well for others.
But a healthy intestinal tract is not a bad idea in any instance.
I have 2 sons with autism and they are thriving from only intensive ABA. There are no studies that support or show these diets work and most of my son's therapists say that the boys who try the diets actually seem to be worse from them as they take away so many things kids want. If there's a true allergy involved, then it would be a different story, but as a health professional myself, there's just nothing out there to support this outside from an allergy.
I think if a child has an allergy or intolerance that it is going to cause them to show increased behaviours basically because they don't feel well, have headaches etc etc. By removing the substance from the diet there can be a reduction in some children in some behaviours. But allergy or intolerance do not cause autism and you cannot recover from autism. By supporting the child for their difficulties eg. speech and language/social communication, sensory differences, therapies etc etc this will enable a child with autism (or indeed any child), to be able to access learning and process information to a greater degree.
I have fibromyalgia and have less headaches/nausea etc on glutein/dairy free diet. My son is on the autistic spectrum and I have had him on the diet then took him off it. I haven't noticed a huge difference to cause me to want to put him back on it. There are other things I have found much more effective. But I understand that as a parent you want to try as much as you can. But, for example, listening therapies such as Listening With the Whole Body have more clinical evidence that they see improvements in all areas of the childs development. The jury is still out regarding diet.
But something I think that may add to the dietry suspicion is that as children on the spectrum are usually anxious/stressed they do tend to have a high level of stress hormones in their body which will undoubtedly have an effect on the chemical interactions going on within the body. Therefore anything (supports, structures, therapies) that will help the child to understand and cope better in their day to day life at home and in school is going to have a big improvement on how they are feeling and behaving.
It's not going to hurt to try the diet, if that's what you want to do. If you do, I'd break it into two parts. First research and eliminate the Casien. It only takes about 72 hours for that to flush out of the system so you should know one way or another with that within a week. Then, if it doesn't make an appreciable difference but you still want to try the Gluten free diet, you can try that but still allow your child any milk products they like. I imagine the diet truly helps with some children but I doubt that it helps all. The main thing is to do your research so that you can make sure your little one is getting good sources of the vitamins, minerals and other nutrients that such a strict diet can impact. Good luck!