I agree with EvaNoel, enroll in an obedience class and see how it goes.
I'm not completely sure but from what I hear Austin doesn't have boundaries and limitations (like cesar says
) and it sounds as if he isn't used to it. An obedience class will help.
Also from what I read, Austin learned that if he barks and jumps at someone he will get picked up. Maybe that's why he keeps on jumping on your little niece.
If it isn't like what I said, I apologize
You have had your baby boy for 5 years now, please do everything in your power to improve his behavior before thinking of giving him away.