Where are you meeting? Is it at their house or at a community place or where? Is it Olive Garden type place or somewhere super fancy like Le Bernadin or Mirazur (look them up--could be somewhere like one of them)? If you do not know the answer to the first question, consider holding off on picking the right outfit for a little while. He has to plan it with his family. Also ask him what he will be wearing so that you two can match outfits but also wear the same type of outfit---
depends on the location, but if it’s at their house or any kind of “casual” setting, a pair of jeans (no rips) or leggings, with a normal T-shirt is usually fine. try not to change yourself up too much, if this relationship is going to last, and you want his family to like you, you have to let know get to know the real you! just be yourself that matters the most.
If he is wearing something for business casual, then wear something that matches that category. If he is wearing something for a "white tie" gathering (low probability but still possible), then you should wear something like a full evening dress. If you do decide to wear a dress, wear something that is your favorite color. You want for his family to think that you look good in it---