Sounds like a good idea to let him know you'll be asking. A similar estate in my family was settled with absolutely no documentation of costs. It was clear that if we had insisted the executor would have had to pull a number from the air, because no records were kept. A real mess. Most of it was outside probate (trusts and beneficiary designations) so the probate court never saw it.
The "income tax" issue above is that the executor fees will be taxable income to the executor. You can't force him to report the fees to the IRS on his 1040, but he ought to.