He walked down the street with his shirt untucked.
In general, your shirt selections should fall about where your crotch is when untucked, for a proper fit when tucked into trousers.
Look for regular shirts in a slightly shorter style if they will remain untucked, showing a little more in the trouser region means that your legs will look longer.
If your pants fit and your shirt is worn untucked, feel free to leave the belt at home.
Unless your boss has said so already, don't assume that you should go to work with your shirt untucked.
They look right at home tucked into a pair of crisp trousers, paired with jeans (preferably untucked for a more casual look) and tucked into long shorts, as on the golf course.