Bruno is proud of his father, who has achieved the rank of Commandant in the Nazi army.  This pride comes out when talking with his friends.  In short, Bruno describes his father as an excellent soldier who has a grand uniform.  Although Bruno had many friends back home in Berlin, he only has one friend after he moves to Auschwitz:  Shmuel.  We are not told about how Bruno describes his dad with his other friends; therefore, the way that the reader knows how Bruno describes his dad should be based solely on his conversations with Shmuel.  

At one point, the two boys discuss what they want to be when they grow up. When Bruno says he wants to be a soldier, Shmuel tells Bruno that he would never want to be a soldier. Bruno gets offended when Shmuel tells Bruno that, despite the fact that Father is a soldier, "there aren't any good soldiers."  Bruno tries to convince Shmuel by talking about Father's "impressive uniform."  

Big things ... the Fury has ... in mind for [Father] because he’s such a good soldier.

Shmuel admits that Bruno does not "know what it’s like here.” Therefore, Bruno describes his father as a "good soldier" with an "impressive uniform."