Comment #13 by vanmacguy is spot on and you should follow his advice.
If you don't trust your admin then revoke his rights and let him go. Find someone you do trust (or learn to do it yourself and be prepared to spend the time doing it).
A sysadmin can read, change and delete any file on the network. They can read any email. You must be able to trust them.
P.S. If he is a power-crazed loon, as you suspect, revoke his rights before you fire him because he'll probably try and sabotage your network in retaliation. Get another network admin to check for backdoors too - hidden admin accounts, VPN tunnels, that kind of thing. Pay a consultant if that is what it takes.
