http://www.allergygrocery.com/search.php
Stefanie,
Miss Roben's offers many allergen free mixes. On the website, check off your allergens/intolerances and many items come up with wheat free, yeast free, egg free, dairy free. A white cake mix came up when I selected "Miss Roben's mixes" in the "Search Through" box. If your son can tolerate the ingredients in dry Jello pudding mix, I always add a box of the dry mix to my gluten free cake mixes to help them turn out moister. Miss Roben's mixes have usually turned out pretty well for me & taste great.
Good luck,
Janice