ooh, that's kind of fun.
Start insisting that he must really miss her, and therefore he should ask her out for his own sake, and just really press the issue, "You should call her up, really, I think you should ask her out!" Even if she's got a new boyfriend, etc. (Point being, you don't need to genuinely think this--you just need to use the opening to make him feel uncomfortable by being "encouraging" about his love life.)
Even better if your boyfriend joins in. Pretty soon he'll stop just to keep you from pressuring him to ask her out.