Most Helpful Guy
"Babe" is his comical term of endearment for his buddy. You are his girlfriend and serious, not a joke. So he doesn't want to call you the same thing. Just ask him to call you "hun". Problem solved.
Most Helpful Girl
Since he's known his friends for a long time maybe, he has gotten real comfortable with them and is comfortable enough to call them babe. However, since you are his girlfriend, its probably different. he's not comfortable enough to call you babe as he is with his friends. Guess it just takes time