If some one is charged with aggravated discharge of a firearm and there's no gun will he do time?

Whether or not a person charged with a crime (which obviously can result in a conviction, a criminal record, incarceration, and other consequences) should defend himself or offer to cooperate with the State is not something that can be properly assessed from the facts you list.

I strongly urge your brother to contact an experienced criminal defense lawyer admitted to practice law in IL (a number of excellent ones list here on Avvo.com) and discuss with the lawyer, under the attorney-client privilege, exactly what information he has; what his involvement actually was; and, what proofs the State will likely seek to use against your brother. An assesment can then be accurately made by the lawyer upon which your brother can base his decision on how to handle this. Good luck.

DISCLAIMER
This answer does not create an attorney-client relationship or constitute legal advice. Given the nature of this website, it is provided solely for informational purposes, for you to use as a starting point when speaking directly with a lawyer in your State. Do not assume that the legal theories I mention that pertain to NJ will apply in your State. I urge you to immediately contact an experienced criminal defense lawyer admitted to practice in your State before making any decisions about this case.