Box 12
Here is the link for the IRS on common errors and common codes to use: http://www.google.com/search?q=irs+retirement+on+w2&rls=com.microsoft:en-us:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7GFRE_en
for all codes you can go to the general instuctions for W-2: http://www.irs.gov/instructions/iw2w3/ar01.html
It also depends on how it is given (part of his benefit package or can he choose to receive it as income if he wants ect.)