Question

where are his keys 可以写成where is his key吗?

Answer 1

Where are his keys?由于主语keys是复数，所以系动词用are，正确。现改为Where is his key?主语变成了单数key，系动词也随之改为单数is，主谓一致，正确。

Answer 2

如果是单数可以
Where are his keys?指两把钥匙或以上

Where is his key？ 指一把钥匙

Answer 3

你这两个句语法都有错误
应该是where does his keys
你把疑问代词去掉还原就是his are keys和his is keys明显你这两个句子都有语法错误