I have slightly different experience in grad education to bring to this discussion. I’m in a PhD program, so I think the entry process is different. For instance, it made a big difference to my acceptance prospects that I applied to departments that had professors in my field who were willing to take me on as a grad student. I might even have been accepted into my current program because of one of the professors pulling for me. After the grad student/department match, I think the most important things in my favor were my application letter, GRE scores/resumé, and my writing sample (equivalent to an art student’s portfolio, I suppose).
But in response to your question, I’d say your son needs to have both a killer application essay and a killer portfolio. I think it would indeed stop him dead in the water if he applied without a portfolio. If it would help him come up with some more recent work, perhaps your son could take 6 months or a year off from the grad school application process to work on some projects. But in any case, I would estimate that having a first rate portfolio is more important than having an ok portfolio of very recent work.
And I’d like to strongly second a previous suggestion that he not only apply to two programs!!! I myself had very competitive GRE scores, college grades, recommendation letters, writing samples, etc., but grad school acceptance rates/competition as well as internal departmental politics being what they are, I was rejected from every place I applied to but one, where I was waitlisted and then accepted. (Strangely enough, the place I was waitlisted I had considered my “safety school”—and yet it turned out to be the best match for me! A strong individual match with a department is as important as, if not more important than, the ranking of the program, IMHO.) Lucky that I applied to 5 or 6 schools! This is called hedging your bets :)
Finally, I’m not sure how important it is in the field of art, but it can’t hurt—I recommend that your son contact professors with whom he envisions himself studying at these schools. Not necessarily to pose questions, but just to introduce himself and make his name and interests known to people who might be reading his applications in the future.
Best of luck to you both !!