define poorly...there are no published standards for this test, so unless your son just can't do a pushup, or runs over 10 minutes for the 1 mile, it won't really make a difference. If your son has adequate SAL attributes the PFT will probably just be a confirmation for the board. If your son is lacking Athlete attributes the PFT score may bear a little more weight. Since this is the last time the PFT will mean anything I would contend that it won't mean much this time either, unless it is really bad. If it is that bad, I'm with agles, don't send it in.
I'm advising applicants to take the PFT this year to avoid confusing the board. Educateme has good advice for when he's ready to take the APFT. All but one of my scholarship winners this year had to take the APFT more than once for me to validate their scholarship. Don't worry about the APFT at this point.
Keep in mind the deadline is approaching for the first board. If you are screwing around with PFT scores now, they aren't going to change much in two weeks. Get it done and submitted. If you don't get picked up for the first board, retake the test and send in the new scores if you think it will help.