Potatoes are not that rare of an allergy, actually. Potatoes are part of the nightshade family so as such, you should avoid giving him other members of that family - Brinjal, Cayenne, Capsicum, Eggplant, Ground Cherry, Banana Pepper, Bell Pepper, Chili Pepper, Green Pepper, Red Pepper, Sweet Pepper, Paprika, Pimento, Potato, Tabasco, Thorn Apple, Tobacco, Tomato. I don't agree that carrots and squash need to be avoided - unless they are causing problems too.
From the little I know, the Feingold diet is often used to help treat/manage ADHD and related issues.