Quarterdeck in VA is a fun outing for all crab loving ages + you're a nice aunt to take your nephew on a crab quest. Here's a Washingtonian article about crab places from a few years ago:
https://www.washingtonian.com/2022/08/18/where-to-feast-on-crabs-around-dc/

If he liked Red Lobster Clyde's at Mark Center in Alexandria is a safe bet and should be able to accommodate a group.

How about Vola’s in Alexandria?

https://volasdockside.com/

My 12 year old nephew is coming to town and his one request is that he tries seafood while he’s here. He just tried lobster for the first time at red lobster and loved it and now he wants to try more. My first thought is crab but we don’t have time to go to Baltimore. I love Joe’s stone crab but it is too fancy. Any good place for this? They’re staying around Alexandria but we live in DC and have a car. This will also be a pretty large group so I’d like to be able to make a reservation.