Good point. I've found a few places that add Soy Sauce to their beef marinade. Same with adding flour to their corn masa. The more Americanized the Mexican restaurant, the worse off you probably will be. Conversely, the more traditional, the better off you'll be (wouldn't be caught dead putting flour tortilla in a taco, or using cheese in a taco, either). That being said, the language barrier.
Best options if you aren't in a major market is probably going to be Chipotle or making it yourself at home. There's plenty of major brands that have corn tortillas. Even gluten-free enchillada sauce. You could always substitute Vegan cheese, its not terrible if melted.
Curious - does your son have allergies to Goat cheese?