Welcome!
Other, much more knowledgeable, people will be along to help anwer your questions, I am sure. But I just wanted to let you know that you have been giving your son what he needs all along. If there were any major problems you had overlooked, there would be evidence of his great unhappiness in school. And from what it sounds like, he seems happy enough. Also, as evidenced by his improving scores over time, he is learning every year, even though it may not be at the pace at which he is capable.
I think you are off to a good start in advocating for your son, and the school seems like they have been working to get him what he needs more recently, albeit none of it during actual class time. Perhaps he needs more during class time in addition to the afterschool opportunities they are offering him. So, whether he needs more is something you should think about. In thinking about it, consider what your values are regarding education, what you want for him out of schooling and life. You've made the first steps in figuring out what he needs now, which is to identify through testing and observation where his strengths lie and where they lie on the continuum (consistently 97th to 99th percentile is quite high). Again, other people will be along shortly to give you ideas on what to do next to help you answer the question of how you tell smart from gifted, but IMO, if his school is noticing and acting upon these scores, then he's something out of the ordinary.
Have confidence that you have been meeting his needs and that by investigating what, if anything, to do next, you are continuing to meet his needs. Even if you ignored or just didn't realize that he needed more, it's never too late. You sound like a loving parent, and that, after all, is what all kids need most.
