Most Helpful Guy
If you haven't asked him for his number then how will you explain why you have his number?
can't you add him on Facebook? Nowadays almost everybody has one and adding someone would be "inconspicuous".
Most Helpful Girl
You don't need to be blunt but you do need to be clear, and by being casual, you can achieve a smooth request.
Say something casual on the lines of "Hey, can I have your number to text you sometimes?" Simple. Then say thanks, and walk away.
At this point, he'll anticipate your text, and you'll have his number.