For a dairy substitute, I would consider buying goat milk. People with dairy issues typically have issues with cow milk products specifically, and goat milk can be a godsend! It's more natural than rice, almond, or soy milk and has more of the benefits that animal milk brings. If you are able to acquire raw goat milk, it actually is the most "complete" food available and the closest to a mother's breast milk to be feeding a child (although to a lot of people it's controversial). I developed an autoimmune allergy to cow dairy at the age of 20 but I'm not lactose intolerant. It's an allergy to the protein casein. I also ended up with an intolerance to gluten just within the past year at age 25. I notice that I do best if I stay away from soybean oils too (like in mayo or margarine). I had blood tests done for years by several physicians and the tests always showed up negative for any allergies or intolerances, so I try to caution people that they shouldn't stop there and accept a diagnose of "nothing is wrong" if they are clearly experiencing symptoms! I was finally diagnosed by muscle testing with a chiropractor, an ELISA/ACT blood test for autoimmune allergies (not widely accepted by the average doctor), and my own trial and error through an elimination diet. I have seen numerous GI doctors and even had an endoscopy at one point to determine if I might have Crohn's or Celiac disease because I was in so much pain. Again, please seek alternative opinions maybe in the more naturopathic arena of healthcare if you don't get the answers you need from other doctors. I suffered through years of invalidation, depression, and horrible symptoms with no hope in sight because my doctors were in denial that something may still be going on since they couldn't figure it out. Plus, when doctors focus on just "allergies" they tend to rule out the spectrum of intolerances that can't be detected as easily.
It sounds like you're already choosing some good foods, and other people had great suggestions! I am in love with rice pastas. They're expensive, but I actually like their texture better than regular pasta now (if I undercook them compared to the time suggested on the label). I also do a LOT with rice. I've had fun experimenting with new dishes and just randomly throwing things together that turned out amazing. We make stir fry a lot with a bag of frozen veggies and any meat of our choice. We use gluten-free soy and teriyaki sauces. I also found one GFCF bread that I really like, so that has helped. There are GFCF waffles in the frozen section of most grocery stories too (well, at least around here they have them). So I'll have waffles for breakfast sometimes and put peanut butter or almond butter on top with some blueberries or banana slices. Very filling and yummy! I wish I had more options as far as yogurts and other dairy products go, but goat products tend to be expensive so I don't get them a lot. There are goat milk yogurts out there that are super tasty, though, and I eat goat feta and other goat cheeses too. I have so many meals that I've come up with, it would take me forever to list them here. But really if you just have solid substitutes for various ingredients, you can make most dishes taste practically the same! That's how I came to have so much variety in my meals.
Oh, and one note about butter... You may find that you can eat butter and your son could be just fine. I stayed away from it and ate margarine for years because I thought it was healthier (which was untrue) and figured that since it is derived from cow milk, I'd have a reaction to it. Actually, I learned from discussions with some friends who own an organic farm that most people don't react to butter because it has milk fat in it and little casein or lactose. It's also better for your body than the soybean oils and other bad fats in margarine. If you want to strictly avoid lactose and casein, though, there is a clarified butter out there called "Ghee" that tastes good too! I do well with either of them. It's hard to find mayonnaise that doesn't contain soybean oils, but I found a canola one. You have to read labels meticulously because I believe there's a Hellmann's canola mayo that still has soybean oil in it.
For cereals, try to stick with gluten-free Chex. I eat the plain and honey flavored ones. I also eat gluten-free oatmeal often. Um, what else... Don't be afraid to eat a lot of stuff you normally would, as long as you're eating mostly whole foods (non-processed) and you have substitutes for other stuff. It's overwhelming at first but you'll adapt quickly, I have no doubt!