I like to take the humorous approach, "Oh, you want babies?! How wonderful! Oh... no? No babies? Okay, condom it is!" Also, if you don't want to ask, you could just put the condom on him before having sex. Some men think that this is incredibly sexual and actually like when the woman takes control of the situation. Or, like that ladies above suggest, you could just demand he wear one or you'll withhold sex or find another man.
Also, just wanted to put in that you should not 'ask' for him to use a condom... You tell him to. No condom, no sex.
Agree make it know it's what you want done if he respects you he will have no problem doing what you want
Don't ask demand and don't give him the option