DS2 is the fussiest eater and only eats tiny amounts- For breakfast he eats weetbix and tinned peaches or cherrios and peaches (both are wholegrain) he eats homemade pizza- you can grate and hide vege under the cheese- once you have made the base a few times it gets easier- you just gotta remember early enough so the dough can rise. Smoothies/ milkshakes- you can shove complan/ sustigen/ protein powder into these and probably could put fine metamusil if needed. We do milk, yoghurt, fruit, and vanilla or strawberry complan.
Homemade tortilline is also a good way to hide stuff and again once you have made the pasta base afew times it gets easier, then you can put vege and meat into the pasta and seal it. (I freeze batches of them put enough in for all the kids in a bag and make lots of bags)
Hope some of this helps