You could always due a system restore to the phone to get rid of iOS 7 and install version 6. I doubt that your grandson used the standard system update feature.
Some months ago I had an alert on my phone to update the OS to 6. something or other. I just ignored it for a while, then my grandson got hold of it and asked me if I wanted him to update it for me. I said yes, as long as you back it up first.
So now in trying to figure out how some of the new things work, I find that instead of OS 6...., I now have OS 7, which is supposedly for developers only at this stage, so how did he get it on my phone??
And could this be why it loses a charge after a couple of hours? How do I fix this?