You can't charge anyone with anyone. Only the District Attorney can file criminal charges (or, as it's known in juvenile court, a delinquency wardship petition).
If the local cops won't do anything about it, you might call the police department and ask to speak to a supervisor.
As someone who represents many juveniles, I would encourage you to consider a couple of things first:
1) Some kids are just plain bullies, and the attacks on your grandson might be completely unprovoked... but many children are reluctant to tell their parents or grandparents the whole story, and you should be sure your grandson isn't playing a role in this before you call the cops again.
2) If you are successful in getting the police involved, there's a good chance the other kid will step on the beatings in retaliation.
Yes someone under 14 can be charged, however you will need the police to file a report and forward that to the DA to charge the defendant. I would recommend talking to that kids parents about the issue of the bullying and take your grandson to self defense classes so that if it happens again he can defend himself. Bullies cannot be bullies when they lose fights.