but my accountant is insisting to me that I erred in this particular deduction
So did you? I think you mean to say you erred in the way you claimed/entered it, because I'm sure your accountant knows you're entitled to it. I suspect you may not have entered it correctly. The start-up expense should be classified as an amortized asset under IRC Code Section 195 with a maximum lifespan of 15 years if the costs exceeds $5000, or 1 year if under $5000. It appears on the SCH C Part V, Other Expenses.
This is covered in IRS publication 535 starting on page 26 at https://www.irs.gov/pub/irs-pdf/p535.pdf
