Not sure where he hit, no blood trail, and not sure the direction he went. Those are all very bad starts. Not to knock your brother in anyway, but it sounds like this is going to be a tough lesson learned in the world of bowhunting for him.
I'm guessing from your description the buck jumped his string, and he hit him high, near the spine. As others have agreed that would explain the falling down. It would also explain the lack of a blood trail. If he hit him high like that then its likely his arrow never entered the chest cavity, but likely the muscle tissue area at the backstraps. If so thats a non fatal hit so long as his spine wasn't damaged so he can't get around ok.It doesn't sound like it did.
This is all based on your description, and hard to know for sure without being there. But your brother most definitly should seek some help looking. I'm thinking his only chance is if he nicked the spine enough he might find him bedded, and get a fatal shot in him. So tell him to track with his bow, and be ready for another shot.

dubuque is about 15-20 mins away from me