Porridge can be made with water! Add sugar to make it tasty, if necessary. And fruits.
Ok, lots to look at there. Boxed cereals are the work of the devil. Sorry, but they are. Very few are good nutritionally (wholewheat my arse
), and even if they are, if a child is displaying your child's symptoms they need to be cut down until you see what is going on. Try the continental approach to breakfasts - if he likes deli meats, you're onto a winner already.
I have HUGE sympathy, if I sound demented it's because I have been there in this house so much, it frustrates me! my issues, apologies.
The explosive poos sound like intolerance of SOMETHING - either milk proteins or gluten, and you are clearly leaning towards the gluten, as you know him best and what he eats.
Re: dietitians, they are great, well educated and very professional, however in my experience if a child is thriving (as yours clearly is) and there is no underlying clear medical condition, they are at a bit of a loss.
Intolerances are notoriously hard to diagnose, as opposed to allergies, as another poster outlined above. They will generally bat the ball back into the parents court, not unreasonably.
White meats, fish, fruits, plenty of vegetables, lots of fruits juices, is a way that worked in this house. If he doesn't like milk, then yoghurts, ice cream, cheeses, frubes, cheese strings, whatever, to keep the calcium and calories up.
I have a bread maker, and there are lots of gluten free bread recipes in the book which came with it. It's great.