Again, I think you have to be careful applying research based on average kids to an outlier kid. While that "gift" of an extra year might help kids of an average or lower ability, high ability kids don't need that advantage since they will likely already be at the top.
I would also consider where you live. A kid who will turn 7 at the end of K (in June) would still be an anomaly in many many places with strong school systems. I don't believe that hitting 19 by the time you graduate is consider "normal" in Pennsylvania. Your DS would encounter many kids who are around one full year younger, which can be detrimental to his self-concept. Letting your DS start school at the proper year will result in him being among the youngest quarter of kids but holding him back may result in embarrassing questions about grade retention by the time he hit upper elementary.
