Both Brazil and the US have allowed dual citizenship for some years now, so he's fine on that front.

The governments of the US might tolerate dual citizenship, or turn a blind eye towards to it, but legally speaking, it's not allowed; you can have allegiance to only the United States.

Depends on what you mean by "legally speaking". The citizenship oath does require allegiance to the United States, but unlike in some countries, there is no law requiring naturalized citizens to renounce other citizenships as a condition of naturalization. So both practically speaking and legally speaking, dual citizenship is permitted.

Depends on when it is done ... for example as a child the rules are different for a lot of countries and dual citizenship would be allowed.