I would add a probiotic to your treatment. When I give antibiotics I give a probiotic also, which often seems to lessen the intestinal impact of the antibiotic, and I continue for a week or two following antibiotics. When I'm treating a single chicken I use a human capsule and mix it with the dose. When I'm treating more than one I mix it in the feed made into a mash with water for all to eat and use a powdered livestock one for that. The antibiotics will kill all bacteria, both good and bad, it can be very helpful to replace the good, which can help keep the bad at bay in some cases.
This is from the Merck manual linked above:
Prevention, Control, and Treatment:
Because C perfringens is nearly ubiquitous, it is important to prevent coccidiosis, especially E acervulina and E maxima infections, as well as changes in the intestinal microflora that would promote its growth. This has traditionally been accomplished by adding antibiotics in the feed such as virginiamycin (20 g/ton feed), bacitracin (50 g/ton feed), and lincomycin (2 g/ton feed), as well as ionophore-class anticoccidial treatments. The move to antibiotic-free feeds has also been associated with markedly increased use of coccidiosis vaccines, resulting in early circulation of mixed Eimeria infections that are associated with the resurgence in incidence of necrotic enteritis. Avoiding drastic changes in feed and minimizing the level of fishmeal, wheat, barley, or rye in the diet can also help prevent necrotic enteritis. When higher amounts of wheat, barley, or rye are necessary, use of enzymes for nonstarch polysaccharides in the feed has reduced the level of necrotic enteritis in flocks fed these cereals. Administration of selected probiotics or competitive exclusion cultures has been used successfully to both prevent and treat clinical necrotic enteritis (presumably by preventing proliferation of C perfringens).
Treatment for necrotic enteritis is most commonly administered in the drinking water, with bacitracin (200–400 mg/gal. for 5–7 days), penicillin (1,500,000 u/gal. for 5 days), and lincomycin (64 mg/gal. for 7 days) most often used. In each case, the medicated drinking water should be the sole source of water. Moribund birds should be removed promptly, because they can serve as a source of toxicosis or infection due to cannibalism.
