In the USA I don't think he would be protected as an International Organization staffer (he is not a diplomat):
https://en.wikipedia.org/wiki/Diplomatic_immunity#In_the_Uni...
"As opposed to United Nations officials, experts on missions for the United Nations, like members of the International Law Commission, Special Rapporteurs, or members of United Nations peacekeeping operations, serve under a temporary and specific mandate. They also enjoy certain functionally limited privileges and immunities pursuant to article VI of the General Convention."[1]
The relevant document is "Convention on the Privileges and Immunities of the Specialized Agencies" from 1947[2].
So, as long as the US doesn't intend to break that convention specialists working for the UN probably have immunity in the US too.
(IANAL, and so on)
[1] http://legal.un.org/avl/ha/cpiun-cpisa/cpiun-cpisa.html [2] http://portal.unesco.org/en/ev.php-URL_ID=48887&URL_DO=DO_TO...
An indication that he might actually have been doing what he was accused of?
His job for the UN was to find out if tunisia is violating UN rules.
In other words, he did exactly what they are accusing him of, just that it isn't spying because tunisia have themselves decided to be a part of UN.