"He could be a suitable person and could handle it perfectly", President Bolsonaro said.
"Eduardo is a friend of Donald Trump's children, speaks English and Spanish, and has long experience with world affairs. It is not nepotism, I would never do that", Bolsonaro said on a live social media broadcast on Friday.
"It is a total scandal ... a flagrant case of nepotism that can not be accepted", Folha de Sao Paulo quoted opposition representative Randolfe Rodrigues as saying.
After Jair Bolsonaro was elected in October, Eduardo was one of his first envoys to Washington, where he met with Trump's son-in-law and adviser Jared Kushner and was spotted wearing a "Trump 2020" cap. Bolsonaro wants to govern in his backyard, extending (the government) to his family.More news: Karnataka political crisis | Trinamool organises Opposition protest in Parliament
He called the move "unprecedent in the history of Brazilian diplomacy" and said he was "certain" the Senate would not approve the nomination.
Congressman Ivan Valente of the left-wing Socialism and Liberty Party said the nomination would be unethical. "An global embarrassment", he tweeted.
Mr Bolsonaro uses several of his members of the family as reliable advisers.
A 2017 YouTube video shows him standing in front of a Popeyes chicken counter recounting his time in the U.S.in 2004-2005.More news: FTC Facebook settlement: $5 billion fine reported for user privacy violations
Eduardo Bolsonaro has said he would like to leave Parliament to become ambassador to the U.S.
Though he now serves as the head of the House of Representatives' Foreign Relations Committee, he has never taken an actual diplomatic job before.
He accompanied the Brazilian leader to a private meeting with US President Donald Trump during a diplomatic visit to Washington in March. But he questioned how powerful Eduardo Bolsonaro would be able to be there. "Eduardo Bolsonaro's nomination for ambassador is unacceptable".
The discipline of ambassador has been vacant since April.More news: Federer denies Nadal in Wimbledon men's semi-final