First, your son posted some impressive scores!
Second, it is not possible to use his achievement scores to indicate anything about growth (learning new material), as they are from only a single moment in time. You need repeated measures to make any meaningful comment about growth. And the The principal's comment overlooks the possibility that he walked into the grade already having mastered those skills.
And, the measures need to have sufficient range to distinguish significant growth, which leads to...
Third, his group achievement scores are all clearly hitting the respective ceilings of the tests. Terra Nova (like most other group standardized achievement tests) usually has about 2 to 3 grades of range above the nominal grade set. And yes, the CogAT is supposed to be a measure of aptitude, not achievement. Given the 99th %ile scores, it is difficult to say that his math should or should not be at the level of his reading. The WIAT scores suggest that his computational skills are slightly stronger than his word calling skills, in comparison to the range of skill expected for end K students. (I realize that the grade equivalents would suggest the opposite, but GEs are notoriously unreliable.) In reality, of course, a 9 point standard score difference is negligible. Whether he is ready for algebra is not easy to infer from the available data, as none of it assesses him in any depth on pre-algebra skills, if at all, and some of it is quite old, compared to his probable rate of learning.
If he's mastered basic arithmetic, why not just try him on the next steps in math, and see how he does? Check on the recommended resource board for a variety of suggestions for curricula. (I assume you are talking about afterschooling, at the moment.) There's no rule against algebra at age 7.
