If your son married after you naturalized, then the petition converts to F-1 and then to F-3 upon his marriage. This way, he would still benefit from the July 2008 priority date.
However, if he married before you naturalized, there does not appear to be any authority for his keeping the July 2008 priority date. If this is the case, then you should file another petition as soon as possible to get the earliest possible priority date.
Answered on Aug 10th, 2013 at 3:25 PM