From around the water cooler at Max's Price Guides:
"Men are simple creatures: a man wants food, affection, and to be the "manly hero" in the eyes of their mate. Allowing the man to propose permits him to be the 'manly hero' rescuing the damsel in distress from a life of poverty (yes, deep down he wants to be the hero breadwinner) and insecurity (yes, deep down he wants to be the family protector). If the women proposes, it takes the 'manly hero' role away from the man."
"The person with the least interest in the relationship will control it. If the woman proposes marriage in a culture where the man usually proposes, then this may just tip that balance of power unfavorably to the man."
"The right guy will be on a knee, with a ring (yes, a diamond ring!), asking, and it won't be a surprise. If you have to ask, you have the wrong guy!"
So think twice before asking him!
(And do help him out by sending him a link to diamondpricebluebook.com so he doesn't overpay for the diamond in that ring!)