A bit silly in this day and age. He should know better.
I called my nieces and nephews little monkeys, one of them happens to be mixed race, never thought anything of it.
Lived in Africa with both monkeys and black people, never once heard a black person called a monkey
But I accept that in this day and age it can very well be seen as derogative and therefore would never use it.
Better not call any future babies in the family piccaninny either now.