well, it's really the guys job to ask. guys take the risk of rejection.
he might not ask because:
a) not interested in you romantically
b) is a big pussy
c) is not allowed to ask you out because he is in a position of authority (your boss or something)
d) doesn't want an office romance
e) is already into someone else
you can probably find out e) and would already know c)
either way, your best strategy is to be a coquette. this means to act hot and cold. pay him some attention for a while, then ignore him for a while, and repeat. dress hot. flirt with other guys.
link good luck