You have offered a range of options to work the case up further, but Dave lacks funds for further diagnostics, and is eager to get going to pick the kids up from school.
Based on your physical exam and history, you suspect the chicken is suffering from some kind of bacterial infection. You prescribe a Chlortetracycline treatment for the sick chicken and ask Dave to contact you if there is no improvement or the chicken deteriorates. With the Chlortetracycline, you also provide a withholding period note.
A useful factsheet on drug use and withholding periods for backyard poultry can be found here.