Hi!
Eczema can certainly be caused corn and citrus, but also by egg, dairy, soy, or any other allergen. I would first think back to when the eczema started and try to remember what foods/formulas were introduced. If you can narrow it down, then I would cut out the suspected problematic food immediately and probably anything introduced after that time. Try to bring his diet down to a very limited number of foods (no more than three maybe? He's still very young.). Once he has cleared up (can take up to a month), then you can slooooowly (every two weeks) add new foods if you feel it's necessary. If he doesn't clear up with the first elimination, then the problem is very likely something that is still in his diet and you'll need to narrow it down more.
While my son's eczema was directly related to food and pretty much only food, you should also look at his soaps and the laundry detergent. Certain scents/ingredients can trigger eczema or be very abrasive on existing eczema.
HTH! I'm sure others will pass on their thoughts and experiences.
Missy