There's a difference between a food allergy and a food intollerance. If you have a food allergy, you will usually have an almost immediate reaction to the food that you are allergic to. If you have food intolerance, your body will have a more low grade reaction that may occur within days of eating that particular food. A food allergy blood test will not tell you what foods you have intollerance issues with.
Sometimes when you change one thing about your diet or what supplements you take, you will have an immediate positive reaction to that one change. A lot of times, it's a combination of things that all work together that correspond with the healing effects that you are seeing.
A really good book that explains it all in a very simplified manner is The Autism & ADHD Diet by Barrie Silberberg. I highly recommend it. I would also recommend the following:
* Make one change (diet modification/supplement) at a time and stick with that change for 3 days to a week before adding something new to the mix.
* After being gluten free/casein free for a period of 3 weeks, do a challenge but adding back a little bit of gluten to your son's diet and track how he does over the course of the next several days. If you see regression or digestion issues, then you know gluten is a problem. If he's doing fine eat gluten again, wait a week or so and add dairy back into his diet and see how he does over the next several days.
* Keep a log of what he is eating, what supplements he is taking, when he is taking them and how his behavior has been. You can find a sample log on the website www.talkaboutcuringautism.org. The log will tell you everything you need to know about what's working and what's not.
If you have any other questions, feel free to PM me and I'll answer them the best I can.
Hope this helps.