I would try a simple fast-forward-to-the-future experiment. This might actually slow him down. Take him aside and ask him, what he's going to do next time he sees her. Then ask him where they'd be a month from now, 6 months, a year, 2 years, 5 years, if he can even think that far ahead. If he can't, maybe he should.
If marriage is anywhere in this period, then follow up with the marriage, and how well he knows her, and how supportive she is and all that jazz. If you see that he's thinking of marriage too soon, ask him if it's really enough time for him to deicede whether or not he should get married. All you try to do, is inject doubt, and the rest takes care of itself. Try not to lead him to a particular path, he chooses the path, you simply become his "reality check" his "devil's advocate" And keep asking about what the girl feels like, and if he is evn considering her feelings.
This should slow him down and bring him down to an earthy view of himself and the girl. This way you won't be blamed for "killing" the relationship.
Here's a quick lesson in awareness.
Awareness 101
After dating the girl once, his awareness is focused to all of her good features. She is now in a pedestal for him. Forcing the awreness on the other side, usually makes him trust you less!!! The method above, of injecting doubt, is one way to blow up his frame of awareness so he focuses on himself and all the other good things that DJs focus on.
So following this arguing, there could be no single "magic" phrase that will help him change his focus, and somtimes even hard evidence, like photos, or video are usually reframed by these persons so it will fit the "image" that he has of her. Insteas of seeing her for who she is, he'll simply go "Well, she must have had good reasons..." or "It must be a setup, it can't be her"
To change awareness, you need time. Not too much time, but a single word, phrase, or paragraph isn't going to cut it. Like I mentioned above, one way is to piece by piece taint his "perfect" image of her. Another one, is to blow it out of proportions so it becomes ridiculous. (ie. "She's so perfect, I would go kneel down to her, kiss her feet and beg her to give me a chance, while she is whipping me with lashes" But be careful, he may just have such a fantasy

)
There is another method, which uses step by step decisions, where you start with a very, very small flaw that he actually noticed. You have him write it down. Then you ask him to expand on that writing a paragraph, or two and then summarizing it. Then maybe expanding again on that summary. This method was used by the Chinese communists to get American POWs to agree with some of the communist views. I think it happend after the Korean war, or the Vietnam war, I'm not so sure. But you can take it all the way from "well there was this little flaw, but it was very minimal" to "Uh, she's not THAT perfect" to "I'm a pretty cool person myself"