Stammering (or stuttering) can be caused by many things, and depending on the cause, could be cured or treated.
Stuttering can have its roots in psychological, or physiological issues. Sometimes when someone experiences trauma, it can cause a nervous tick that results in stuttering. When that happens, the cure is almost always: therapy. Though sometimes, if the trauma is too severe, it can be treated with medication.
When it's a physiological problem... there are a whole score of causes. In a nutshell, sometimes there's a problem with the brain or nervous system itself. Whether it's Tourette's, Cerebral Paulsy, a stroke, or maybe something less severe like head trauma or a long history of drugs. Though if the root cause of stuttering comes from something like this, you can usually tell easily.
If you're not sure why your kid is stuttering (and it's obviously not a horrible disease or an injury), try talking to your kid. Maybe they're just nervous, or need more practice speaking. Try to help them boost their confidence, and that might solve the problem.