My dog has black fly bites!?

Last night my dog went missing for about 4 hours. When he finally returned home he had multiple abrasions around the bottom side of his body. He also had some weird looking red soars all over. Took him to the emergency vet and they clipped and cleaned him up. They decided the soars were black fly bites. They forgot to shave one of the patches though and when i pulled his fur back i noticed really small white rice looking specs on the sore. Anyone have any advice on what to do about this besides bring him back. I already paid 500 for his last visit, and when i called they didn't seem too worried about it. They said that white dots on the bite are normal but these things just look weird