I was in Tai Kwon Do for many years and, regrettably, for personal reasons, had to leave the school at a time when I was preparing to test for my brown belt. My son was, at the same time readying to test for his black belt. I have, for many ... many years, missed the participation and the training of body and mind. For Christmas last year, I gave my son a two-year program at a school which teaches a mixed martial arts program - No, not MMA competition training (though they teach that, too) but a combination of disciplines from karate, tkd, jiu jitsu, and others you may or may not have heard of. As a treat, and because he gave me a 2-4-1 (and free uniform) deal I could not refuse, I enrolled as well.
The head master and owner of the school is a martial artist in multiple disciplines, has been teaching for 40 years, and is in the Karate-Martial Arts Hall of Fame. Can't beat that for a deep well of knowledge! He is a patient and understanding teacher and an incredible father figure to all of his younger students, and often talks to them about their school work. He has even been known to comment to a student that their school work is just as important as their martial arts studies. He encourages them and asks them about their grades and their relationships at school. He also constantly reminds them that martial arts is for self-defense and not for them to take to the playground and conquer the school bully by becoming the next school bully.
How familiar are you with Tai Kwon Do and martial arts in general? Many schools have different approaches and most especially for younger children? How long has your son been in this class/school? Is it possible your son's instructor is just trying to get a feel for his students' mobility and degree of maturity before he delves into more seriuos martial arts? Perhaps he is just trying to ensure all of his students are limber enough before he starts putting them into a more structured program. Some schools have a set schedule where, from this grade to the next is X months/weeks/etc. and they will essentially "sell" you a black belt in however long. Other schools, like my current one, have set schedules for testing - once a month - but YOU are the one who decides when you feel you are ready to test for the next grade.
Go to your son's sinsei/instructor, preferably after class some night when you can be reasonably sure he'll have the time to talk, and ask him if he's got a few minutes to talk to you about the class and your son's progress. Explain to him your (and your son's) concerns about the progress he is (or is not) making. Ask him if there is a more focused class to which he might be switched. Although it is something that probably should have been addressed before you enrolled your son in the classes, you might want to talk to parents of other, slightly higher ranking students about their feelings on the subject.
The important thing is to not go into this as a confrontation. You are only seeking to be educated and become better informed about what your son is learning and doing in class. Convey to the instructor your son's desires and expectations as com0pared to the instructor's expectations. What kind of timeframe might you expect for learning forms or stages of forms? What kind of time frame does he average for testing for the next belt. And be aware, too, in a "good" school, just because he is testing for advancement, does not mean he will achieve his target. He will be judged and, if he does not appear ready, he will not be promoted. Also, you need to understand that you have two roads you can follow. One road will target getting the black belt. The other will focus on the learning that belt signifies. Don't be over anxious to move him through the stages. Remember, it is not the color of the belt that is important but the degree and type of physical and mental learning he achieves.
I have seen an orange belt from one school who can outspar and out-perform a brown belt (3 grades higher (or 6 in some schools)) from a karate mill.
Discipline, respect for self, respect for others, coordination, mental as well as physical agility, self-confidence. These are the types of things you need to focus on. If, after conferring with his instructor, you do not believe your son can achieve these goals at his present school, it may be time to look into another school ... and research it more carefully But make sure you have a clear concept of just exactly what you want your son to take away from this school and his lessons.
Good luck. Keep me posted.