First of all he needs to contact an attorney and the American consulate in area where he is. All of those factors will matter in a Brazilian court, but like in the US, courts will usually side with the mother unless she’s selling crack or something. I believe (not sure here) that Brazil and the US have reciprocity with child custody cases, Brazil will respect an US court decision and vice versa. So get a lawyer and find the best solution.

I assume by the question the child was born in Brazil? if so you will need to go threw the Brazilian courts, highly doubt if the mom fights it that Brazil will allow you to take the child to the USA