I agree. Don't wait to pursue the 504. I don't know about the tests you reference. I'll just add that if the school is willing to test him, let him take the tests and see what happens. It sounds like it's a program that would be interesting but it's not a programs worth getting either of you stressed over if he doesn't make it in. He does look borderline based on the scores above but that doesn't mean anything. I was told when my son was 6 that he was "barely gifted" (by the GT coordinator) and he eventually qualifed for DYS.
I'm also guessing that if he doesn't do well on the testing this year, he'll have the opportunity to take it again next year when you have the 504 in place.
By the way, what types of things are you asking for in the 504?