I was born in 1980 and my Mum and Dad were married before my bith. My Mum is British and my Dad American. My Brother was born in 1991 - yet he has the US passport!!!
I doubt there are any loopholes for me - I have a greencard which I am grateful for.
Your brother was born after the rules changed in November 1986. When he was born, then as long as your father had lived in the U.S. for 5 years, at least 2 of which after age 14, then your brother became a U.S. citizen at birth.
In case your brother isn't aware, he needs to register with Selective Service.
It's mandatory for male U.S. citizens between 18-25 and there are consequences for failure to register. http://www.sss.gov