When my children have taken the ITBS I have insisted that it be given at least two years above grade-by-age level. You can use the ITBS (as a guide) for placement if you do this by finding where the kid scores between the 90th and 94th percentiles, which indicates mastery of that level's material.
Unfortunately, 99s down the line don't tell you where this happens. If your child is just barely at 99 (and wouldn't also earn a slew of 99s on the next level up), then he likely would be at the 97th-98th percentile for the next grade up and in the low 90s in the grade beyond that. At least that's how it's worked out for several years around here (for two kids over multiple levels). The thing is that it is important to confirm this with higher level testing.
The grade equivalent scores they give you simply mean that an *average* 12th grader (at the 50th percentile) would get the same score your son got. This says more about the achievement of average high school students than about your (or my) son's abilities. Average high school students are not mastering high school level material--more like upper elementary material.
