I have a white food bandit too RB - I understand the frustration, especially since he's little

What about smoothies - made with low fat milk but bulked with either raw egg or high cal powder?
Ravioli?
Going to sound silly but frozen vege uncooked - my little guy lived solely off frozen peas for 2 weeks at about that age - every kid I know likes them or frozen beans

Another thing I found worked a treat was eating somewhere different. I know the high chair is well contained, but sometimes the sense of adventure gained by changing the eating location is continued with the willingness to try food.