Be very careful about using medication aimed at a particular species on something from a different species. They can have a radically different effect. An example was a neighbour here who accidentally used a sheep wormer with combined flukicide on his sheep dog, which died immediately. The wormer itself was ok, but not the flukicide.
I would check him every day, check thoroughly for any more maggots and eggs and remove them, spray him with something to overcome the poo-ey smell which attracts the flies, such as Citronella, and keep him as immaculate in the vent area as you can. It's best to find eggs before they hatch, and destroy them, as they don't do any harm at that stage. Your aim is to prevent them hatching.
Can you also keep flies out of the area around him?