My only rule for DS is that he can enter as many competitions as he likes if he is able to give it his best (effort and time) and if it does not mess up with his extracurriculars (like music, sports etc to which he has made commitments).
He is turning out to be an aggressive competitor and seems to take winning for granted, so I encourage him to enter competitions that are too high in level for him in order for him to see what other kids are able to do and to experience losing so that he can learn that competitions are about learning and not about medals.
My opinion is that if your son has time for the competition and is willing to put in the work needed, go for it.
