26 USC 6104(d) says otherwise. Read it.
You still have no right to just demand that he just give his books to
you.

You obviously didn't follow Dieter's suggestion to read the Code. I'll make
it easy for you; it's pasted below. All Dieter needs to do is walk into
their office and demand to inspect the documentation. And to make it really
easy for you, I capitalized the relevant phrase. When it comes to tax law,
Dieter knows what he's talking about.

26 USC 6104(d). Public Inspection of Certain Annual Returns and Applications
for Exemption.
(1) In general.--In the case of an organization described in subsection (c)
or (d) of section 501 and exempt from taxation under section 501(a)--
(A) a copy of--
(i) the annual return filed under section 6033 (relating to returns by
exempt organizations) by such organization, and
(ii) if the organization filed an application for recognition of exemption
under section 501, the exempt status application materials of such
organization, shall be made available by such organization for inspection
during regular business hours by any individual at the principal office of
such organization and, if such organization regularly maintains 1 or more
regional or district offices having 3 or more employees, at each such
regional or district office, and
(B) upon request of an individual made at such principal office or such a
regional or district office, a copy of such annual return and exempt status
application materials shall be provided to such individual without charge
other than a reasonable fee for any reproduction and mailing costs.
The request described in subparagraph (B) must be made in person or in
writing. IF SUCH REQUEST IS MADE IN PERSON, SUCH COPY SHALL BE PROVIDED
IMMEDIATELY and, if made in writing, shall be provided within 30 days.