Practicing for aptitude or cognitive ability testing is generally not recommended. Your child sounds like a bright and capable young person. This does not necessarily mean that GT placement would be the best or only avenue for addressing his learning needs (whether or not he receives qualifying scores). Have you spoken to his classroom teacher about differentiating instruction in his current class? Creating a higher math group? SSA (cross-grade placement for math)?

...pronounced like the long vowel and first letter of the alphabet...