Looks like somebody's done his homework about patronuses! The spell you cast creates a shapeless silver shield between you and the creature. YOur incorporeal patronus is strong enough to halt the dementor but doesn't drive it away completely.

The dementor is stopped for now, but the old man still needs your help. He stares straight ahead with a blank expression, still babbling. He also begins to shiver violently. You'd better do something to help him and quick!