I feel for you. We have one also that is ‘not a good eater’, only it is a psychological problem as well as being picky. She has a noise phobia and if she hears one of her trigger noises she cannot be persuaded to eat. Chez nous, we muddle through with roast chickens, ground meats and cut up chicken/beef liver as extra topping. I put hot water on kibble, mix in a couple of big spoons of canned food (tripe is a favourite) and the shredded meat. Sometimes sardines work as well. Also, she seems to like toppings that have a bit of extra fat in them such as cream or high test yogourt.
And then we have the reverse problem with our lab cross aka the cleanup crew. He is always poised to swoop in and “help”…
Good luck with Monsieur Jeter!