Grade 5 Theory at age nine, I would have thought, is exceptionally good. Do you know what pass mark he got?
You say you 'are non-musical'. Would it not help and encourage DS if he taught YOU to play a bit, and you understood more of what he is doing?
And if he plays to Grade 5, the more practice and the wider variety of material he tackles, the better. It is the PLAYING that is the important part, and there are a lot of on-line music sites that may help and inspire. You can also buy the ABRSM pieces for higher Grades, without actually intending to sit the exam at this time. Also learning the more advanced Scales and Exercises will be useful for future reference.
Eventually, the better he gets and the higher the level of difficulty, I agree he will need a really good teacher to cover the finer aspects of technique and interpretation, but for now I think he needs to play as much as possible.
Does he get an opportunity to play at school? For Assembly, or accompanying other solo instrumentalists? Do you have a good music shop near you, that sells pianos? Some dealers put on demonstrations for the public, to advertise their products. Our local Yamaha dealer used to run little concerts by their school students.
And he also needs to LISTEN to as much piano music, of all genres, as possible. I give you a link to Russian pianist Daniil Trifonov, and if you want other styles, let me know as I have a library of styles:
daniiltrifonov.com/