KADmom,
Your DS reminds me of my older DD13. She was TAG identified since 1st grade but there was no strcutured acceleration in elementary school. When we moved to new school in 6th grade, the TAG class Math students are learning bottom half of 7th and 8th grade, so basically she skipped a year and half. Her struggle started near the end of 1st semester. We wanted her to unskip but she pleaded us to let her stay in. (We thought her younger sister was breathing down her neck (they both got the same score on EXPLORE) and she needed space). There were some gaps and after 2 years and some, she finally caught up with her peers (or may be because Geometry is easier than Algebra I).
I would give him the option of unskipping but reminds him that he was good in Math and that's why he got to skip in the first place. Once he fills the gaps, he would be fine. If you don't have the textbook, here's a good resource for Math.
http://www.mathopolis.com/questions/skills.php?year=6
