Question

He doesn't know next 1how to do 2what to do next 3how to do it

Answer 1

答案是：3。
因为在这里，what是代词，可以单独作do的宾语，所以可以直接说： what to do next.
但how 是副词，不能作do的宾语，所以要添加it 作do的宾语。