Well, what do the writers (Orci/Kurtzman) say? Is the boy Kirk passes his brother or not? And it's possible he is, and they just named him Johnny in oversight. Mistakes happen.
Originally Posted by Maverisms
The movie is officially silent on G. S. Kirk's existence or lack there of
While true, George's name was dubbed out and replaced with "Jonny."
George Kirk may still exist, and it is clearly the intent of the writers, Executive producers (who are also the writers) and director that George does exist. Thus, James (alt) is not George (alt) and was not accidentally mixed up or anything like that.