Because of how the credits work, he will be able to use another band to use a meal credit. Since your adding him to the reservation once you arrive, I'm not sure if he would be able to use one of his bands but my gut says no.
request a card from the front desk for him if you can't find the band, that's free. They may also just give him a grey band as he was added to the reservation (assuming you and him are ok with grey)
I didn't even think about requesting a generic grey band for him. That would totally work and he won't care about the color. And he can use his sister's old band for meals if his does not link to the dining plan. Thanks again for your help!