Elimination of artificial colors and sodium benzoate seems to make a significant difference for a minority of children. This is about the only diet thing that has been confirmed by reputable science.
Just fucking stop.
It's your kid. It's your life. It's your plan, it's your pain, and do whatever the fuck you want. But recognize that when I say "my wife sees this a lot" and "diet can help" the rudest fucking thing you can do is whip out witch doctor claims.
There is no aspect of diet you can't change, you can't experiment with, you can't try out. I got friends whose kid will starve rather than eat foods with color. They've spent the past six years going to a therapist twice a week. Know how much "reputable science" there is behind that? fuckall. So a lot of it is n=1 experimentation to see what works for your kid.
I mean, seriously. Go stroke your fucking chin in the corner for the rest of your life and get all uppity about "gypsies curses" and shit I give no fucks. I hereby solemnly swear I will never fucking offer you advice again.