"Skills acceleration" is quite normal for GT kids. Part of the problem is that elementary levels are rather arbitrarily divided into 6 levels (K-5). Both my 5th graders jumped several grade levels in a single year in reading in early elementary. DS probably jumped about a half dozen reading levels in less than a year while DD jumped about four reading levels within that time frame.
I am sure that your DS is really ahead in math, but I would not conclude that he is 8+ years ahead based on WJIII "grade equivalents" because their standards appear much much lower than standards/curriculum at decent schools. Personally I have to say that the WJII spits out rather goofy grade equivalents unless you have a child who is within a standard deviation either way. You have to ask yourself whether your 1st (or 2nd?) grader is ready for the 9th grade (or 10th?) math curriculum in your district. If not, then start at a level where your DS has clearly mastered all the standards. Many districts provide online an overview of the curriculum/objectives for each grade. Start at the next grade up until you find where your DS places, then approach the school for assessments to qualify for an acceleration a grade lower than where you think his knowledge/skills end.
