I'm happy that you have asked me to assist you with your child and hope that you can benefit from my many years of helping parents like you.
I do not recommend dietary supplements like creatine for a couple of reasons. First, it can be dangerous as the kidneys have to excrete a bigger load of waste products (urea) when excess protein is consumed and second, there is no evidence that creatine actually improves muscle growth or activity anyway.
Not knowing what his height and weight are I can't tell you where he is on the standardized growth chart but it sounds like he has not hit puberty yet and thus his muscles are not really primed to get stronger.
I'd be happy to take a look at his growth data if you want to submit it here but the major determinant in his growth is his genetics and his growth is affected by parental height and age at onset of puberty, which is also hereditary.
Here is information about creatine you might want to take a look at that supports my recommendations:
Please let me know if you have any questions.