In your case, I would furthermore gamify it: I bet you can't figure out what I added or did differently!
And then I have the other kid. He will refuse to participate in the game. I keep the pressure on though. That means he's always exposed to foods outside the comfort zone without too much pressure. But efforts at subterfuge or psychology almost always backfire with him. So I keep all the cards on the table.
"This is a broccoli piece. You have to taste it or else {bribe}".
I don't have all the answers, but we've tried a lot of things with him.