For a young person, everything major they do changes them somewhat.
College changes most people.
Marriage (or a serious, committed relationship) changes most people.
A stint in the service (combat or non-combat, active duty or reserve) changes most people.
TIME changes people.
Will your brother be the same when he comes back? Unlikely. Will it be better or worse? Hard to tell, but typically he will be better - although it's most likely they will be some good and bad.
However, you will also be different when he gets out as well, because time, experience, college, etc... will have changed you too.
This is the Pub. Leave common sense at the door.