We recently ate at BD’s Mongolian Grill, and it was a good experience. The location we visited (Independence, MO) had a special grill in the back that is thoroughly cleaned between uses, especially for allergy sufferers. I requested that they cook his food only in water, and they were able to give him food from the back that had not been on the line (so it wouldn’t run the risk of being contaminated).
If you decide to go to this restaurant, be sure to still call ahead to make sure they have a grill in the back. Also double check how they clean the cooking surfaces/utensils, and so on. I recently went to Hu Hot (I thought it was pretty much the same restaurant) and it was not a good experience.