Food is always good. My bf loves chocolate, so I got him a few gourmet chocolates for his stocking this year.
Other ideas for guys' stockings are small tools, gift cards to Blockbuster or Media Play, or a gift certificate to a casual restaurant (just $5 or $10). A Christmas ornament that symbolizes a sport he plays is also a good idea. (I got my bf an ornament of Santa playing soccer last year.)
Hope this helps!