I think you should apply. Like Grinity said, the worst that can happen is that they ask for more info, and then you can just supplement with whatever they ask for.
Note that the DYS rules are not set in stone. I have noticed in my reading that even with qualifying IQ and achievement test scores, if both are at or near the cutoff, Davidson sometimes asks for more information. On the other hand, they've accepted at least one candidate in the past on portfolio alone. Maybe the IQ test scores, which are on the high side even for DYS acceptees, will help to mitigate the slightly low achievement score, and he'll be accepted without a portfolio. Hence I certainly wouldn't delay to get a portfolio together.
On the other hand, you may already have some things which would work as portfolio items, in which case you can save time by sending the portfolio with the initial application.
This page has info on some portfolio items that have worked in the past. Note that it really wouldn't take long to get some of the listed items together, especially videos of your child talking about his areas of interest.
