Well, perhaps he is waiting to surprise you with one.
Don't let this become an issue between you though. As you are the one who asked him, perhaps he is one of those individuals who never seem to get around to doing stuff, until prompted. He must want to marry you, or he would have said no!
Maybe a subtle hint next time you are walking past a jeweller, about how when you get a ring you'd like it if it was a solitaire/cluster/diamond/ruby/emerald/simple/fancy/white/yellow gold/platinum type ring that would have a matching wedding band, and would he like matching wedding bands....
