Gosh, what a lot to contend with, though I expect a relief to find out what the problem is after so long.
I have by choice decided to give up dairy so am starting to dabble in the alternatives. I did some reading on soy and tbh I personally wouldn't feed it to a young boy in anything other than small and occasional quantities. There's a lot of conflicting information, and it seems it has estrogen mimicking properties amongst much else, so I would be wary of it. Rice milk is apparently not suitable for very young children due to too much trace arsenic.
So I would veer towards almond/coconut/oat/hemp milks if I was feeding a child, and probably try and rotate round them a bit to get some variety, and find out what the child liked.
I have found the supermarket nut etc milks are thinner than I would like, so I am very tempted to try making my own. There's loads of recipes on line. It seems homemade almond milk, and home made cashew cream are very popular. Both are also featured in the new River Cottage light & easy book which is entirely composed of wheat and dairy free recipes so probably well worth buying for you! Light & Easy
If he doesn't like the milks with cereal you could try using them to make porridge/fruit smoothies etc instead where they are less noticeable?
I've brought a pack of oatly oat cream to try, which I'm hoping will work for creamy sauces/stroganoffs, not sure if that's the sort of thing your looking for?
I've heard people say Vegusto is a very good vegan "cheese" but I've not tried it, and I think you need to buy online.
Something else to consider in the cheese line is nutritional yeast flakes. They are very rich in B vitamins and can be added to white sauces (obv made with dairy free milks) to get a mock cheese sauce. And people say they are fine to sprinkle on a pasta dish a bit like parmesan. I've ordered these from Amazon and am waiting for them to arrive, hoping they are nice!, there's lots of suggestions for use if you scroll through reviews. I'm intrigued by the idea of mixing them with tofu to make a kind of cream cheese and may have to try that.
Good luck experimenting, and I hope someone considerably more experienced comes along soon!