Yes, PDD-NOS is a spectrum disorder. It basically means that he has some features of classic autism but not enough to qualify for the diagnosis of autism. When the DSM in the states is rewritten he will likely be diagnosed with a language pragmatics disorder which essentially means that he can misinterpret language unless it is specific and concrete enough for him to grasp it's meaning. So basically he has a high functioning spectrum disorder.
We have used feingold. I have a friend in Australia who eats a low salycilite diet. It is absolutely doable. I printed out charts of the salycilite content of various items. I then balanced our meals accordingly. She describes her diet as a returning to the way our ancestors ate....eating fruits in small amounts in season, eating only a bit of nuts and nut items, and having a diet largely of roasted vegetables, rices and cooked meat.
However, it wasn't what really did it for us, maybe it will be for you. It was GF/CF. I go on every diet my son has ever been on because who likes to cook twice?...yes? So, I am aware of what both diets did for me as well. The low salycilite diet most definately helped me balance my mood. I was less irritable, warmer, calmer and happier. It did the same for my son just in a more noticeable way. However, Gluten and Dairy, as well as other foods high in glutamate, like tomatoes, made his and my thinking cloudy. Just for him it was more extreme then me. He would get fuzzy headed, impulsive and he was on "high gear". I didn't really notice until I stopped eating them just how much BETTER I feel when I don't eat them. You try it...you might find the same thing.
Of course, it is tricky to read labels. Feingold's website is cautioning you not to trust what you read. Manufacturers hide ingredients in general wording, like "spices", or "natural flavors". I believe your best bet is to cook your own food from scratch until or if you think it makes a difference for your family. It is also less expensive to cook rather then buy packaged food.
When we went on GF/CF diets, the most recent being now for a year and a half, but on and off since he was 15 months old, we saw an immediate difference in his mood and behavior. It fortifies you to continue. We went "cold turkey" as you say, and explained it to him as being a diet that some doctors think will help you think better and be the best student and friend you can be. I won't lie, it is HARD, mainly because of the feeling of deprivation, even a "why me", "why my child"...but so many children now are on special diets or are allergic. You'll find that your child isn't the only one at all.
I also disagree with your OT. I was a social worker/ child therapist in my "other life" before being a mom full time. I used to say a great many things to parents I shake my head at now...before I walked in my "mom" shoes. Your OT knows a great deal, I'm sure, and can help you manage your child's sensory input but her knowledge is limited by her life experiences and what she has "read" or learned as conventional wisdom. Again, trust yourself, and think of this a journey and take heart....it is a long journey, where you will try many things to help your family, some with greater success then others. You have been given a gift in some respects. You already think outside the box. Think about what you know about your children and try to help them. And don't give up easily. I had my son on the GF/CF diet on and off for years. I'd always take him off because i'd feel badly that it made him different and he was missing out on yummy foods. I also thought that I would help his gut heal and he could eat the foods again. So, I'll tell you, I think that that was my biggest mistake...not being clear at the beginning that if it helps, stick with it!
Finally, I realized that he was already different and that was his life lesson. He must have chosen it. Why should I feel bad about it. I needed to accept it. And, he chose me to be his mom to learn that he could be strong in who he is and be clear and sure of it. I also saw that it was more for him then just healing his gut. He had a difficult time metabolizing glutamate (gluten and casein are high glutamate foods). Amy Yasko (another one of those alternative voices) believes that an imbalance in glutamate/gaba receptors can occur soon after birth due to a deficiency in vitamin b12....You give the brain a chance to rebalance the reception, so to speak, when you supplement with b12 and allow the plasticity of the brain to rebuild.
So, i've gotten to another place with it, where I accept the reality of his body and what it is dealing with, and I cope with it...while I am supplementing him with b12, I hope that he will one day be able to eat larger amounts of glutamate without becoming a "space cadet". However, right now, he is doing so well, I am happy and feeling very safe here with him at this place. I would have never thought i'd be happy with so much dietary change but it is so much better then him hiding under his school desk, having temper tantrums like a 3 year old and being unable to remember what we just said to him.
Do it your way though. I found it easier to get the adjustment out of the way and jump in feet first, saying, "IT"S FOR HEALTH and a strong body" and within a couple of weeks we all saw the difference. Take it slow if you must, but, beware of cravings, yours and theirs and know that you won't see a difference until it's all cleared from the diet. In a few months it will become a way of life.
GOOD LUCK and good health!