aunteveil - the Sunderland protocol is explained here
basically, there is a urine test available, and they test for the incomplete breakdown of casein/gluten proteins.
the incomplete breakdown leads to absorption of inappropriate levels of proteins etc through the gut, and also their by-products.
dd1 tested off the scale (and it is interesting to see that your ds1's issues are autism-spectrum related. I think at one time Sunderland were theorising that up to 80% of people with ASD related disorders may benefit form their research)
as with anything, it is trial and error. we got dd1's urine tested, and she tested off the scale for gluten intolerance. not so for dairy, but dh has a dairy intolerance anyway, so maybe hers is linked ot this, rather than the specifics that Sunderland look for.
basically, she cannot have any:
gluten (wheat, oats, barley, rye) - thisis an absolute. coeliacs often have a tolerance level, and so a lot of products labelled as coeliac friendly are not suitable. dd1 cannot have any AT ALL.
dairy (specifically casein, the protein found in dairy products. some people find their children are able to tolerate A2 milks (such as goat, buffalo, etc) - the rpotin is slightly different.)
sweetners - a big no. same for flavourings (even "natural" flavourings and preservatives.
also no MSG (which is where the no natural flavourings come in. MSG can be both chemical and natural (often made from pear skins, i believe))
it is hard to get your head around to start with, but then it dawns on you that if you use just "real" food, nothing processed, it is a lot easier to exclude everything. it makes days out etc trickier to begin with, but you soon get used to it, and pack snacks etc.
oh, and a lot of children who follow gf/cf diets are unable to tolerate apples and citrus as well (I know this is the case for dd1)
nutritionist wise, I can recommned nutrilink
I have used them with dd1, and they take a nice gradual approach to things. not gung-ho with invasive tests, and a slowly-slowly approach to finding out what may be the issue. absolutely thorough, though.