Probably many babies at 6 months of age are not allergic to wheat, but some babies are, so it is recommended to delay the introduction to avoid the risk. Some other highly allergenic foods are peanuts, tree nuts, soy, seafood, fish, eggs, milk, kiwi.
BTW, when you start a new food, just give him 1 teaspoon on the first day, observe (skin problems, respiratory problems, vomiting, diarrhea...), gradually increase the amount if he is fine with the new food. Wait for several days before introducing another new food.