This can be a lengthy process, here are the steps you and your veterinarian need to take.
1) Shave the area around the wound, it needs to breathe and be kept clean, having all that hair in the area will only make things worse.
2) Capstar is a good medication for maggots, it needs to be given rectally and not orally. Injectable ivermectin can work as well however there can be some serious side-effects with it.
3) With sterile hemostats/forceps remove visible maggots from the wound.
4) Flush wound with a diluted betadine flush (betadine is cytotoxic so needs to be heavily diluted). Scrub around the wound with gauze soaked in chlorhexadine.
5) Continue with long-term antibiotics until issue is resolved.
6) If he's an outdoor dog he needs to be relocated to a clean, indoor environment where no flies can get at him.
I haven't seen a maggot case in years, I will update with more tricks once I get back to work on Tuesday October 10, will ask the DVM.