It's not an age, per se. It's his developmental level.
A few numbers are good, no matter how old you are! (You can tell I play!) But if she's writing in lots of numbers, ask her if she would put just a few fingering numbers on some pieces so that you both can ascertain whether or not he really needs them all.
More than anything, you should work with your son to make sure he actually can read music, and that he's not memorizing how to play it without understanding what's on the page. If he can't do that, he can't learn more complicated music. He should be equally comfortable in treble and bass clefs. As he progresses, he will learn different keys. And in order to do that, he must be able to really read the music. It's like the difference in just knowing the alphabet, and being able to read words. And more advanced playing is the difference in being able to read individual words and being able to fluently read sentences. That's when piano playing can reap the really fun stuff!
Make sure that he has some fun pieces with songs he knows, and not just what's in the books. If he doesn't like technique exercises, bribe him. Tell him if he spends 10 minutes on his scales (when the teacher gives them to him), that he gets a special treat. Figure out something that makes sense for you both. Without the technique, he will never be able to effectively play "fluently". And that good technique also includes holding his hands properly. The worst thing to let a student do is have poor hand and finger technique. Some of the stuff I've seen kids do, ugh! Like dropping the wrist down almost on the piano itself, or the wrists too high, or fingers out straight, or fingers sticking up all over the place. These are normal for little kids whose fingers are still developing muscles, but it's a process to learn the proper hand technique and he has to work on it with the teacher's help. You should watch the teacher and listen to how she tells him to do it. That will help you to help him, too.
Playing with clay will help strengthen and build muscles in his fingers and hands. (Not play-doh.) Try that.
Hope this helps!