Your response to the replies you have received so far seems rather defensive, apparently because the replies didn't fit what you wanted to hear.
Nationality and ethnicity are two different things, yet you are conflating the two in your own question, so it's not surprising to me that you're dissatisfied with people's replies.
"He was born in Brazil, so technically he is Brazilian, but if he were to describe his nationality..."
If he were to describe his nationality, he would say he's Brazilian, because he IS Brazilian, whether you wish to term it "technically" or not. Nationality refers to a person's citizenship, the right to hold a passport. The Oxford English Dictionary defines nationality as "the status of belonging to a particular nation" -> in this case, BRAZILIAN.
"If he were to describe his nationality by using where his parents were from (German father/Brazilian mother)..."
Again, no. His nationality is Brazilian, because that is where he was born and raised. He is a citizen of Brazil, a holder of a Brazilian passport, therefore his nationality is Brazilian.
In this example, the German part is his ethnicity, NOT his nationality. If he had been born and raised in Germany then he'd be a German citizen, therefore his nationality would be German.
OED definition of ethnicity: "racial, race-related, ethnological, genetic, inherited."
If he wishes to describe his heritage, or ethnic origin, then he would say he is half German and half Brazilian -> or half Brazilian and half German. There is no need to get hung up about the order in which he says them, as there are no rules or laws governing this. He can say them in whichever order he pleases.
There are no protocols or standardised responses, no laws or social conventions that people must follow, to describe their own ethnic origin/heritage. They can describe themselves in whichever way they prefer. The father's ethnic origin or nationality doesn't take precedence over the mother's, or vice versa.
Real life example: I have many friends of mixed heritage so I'll simply pick one. My friend's mother is a British citizen, white English. His father immigrated to the UK from China and is now a British citizen. My friend is simply British. He was born and raised in the UK. His nationality is British. He has British citizenship, a British passport; he is British. If he wished to describe his ethnic origin he'd say what his two halves are, ethnically (i.e. English and Chinese). In the UK we don't use any of these hyphenated terms. He's simply British.
So to sum up, "How to describe nationality of a German-Brazilian.?"
The simple answer is: his nationality is Brazilian.
Common sense and the OED.