Fogo de chao or any meat-heavy restaurant would be good. Fogo de Chao is memorable but it is a chain with some locations in Ohio so it might not feel speicial. Old Ebbitt is really good at dealing with food allergies and special needs and it is a DC institution so that would be a good and special choice, but maybe a bit fancy for a teenager. Any steakhouse would probably be a good option. Farmers Fishers Bakers on the Georgetown waterfront is also good with allergies.

Another idea might be to take them to the seafood market at the Wharf and cook at home, if he eats fish? Or take them to a seafood restaurant, but I don't have a specific allergy-friendly recommendation for one.