He wants to know --- ask about Father Christmas.

D。 whom to 这是“特殊疑问词+动词不定式”的结构,通常可以改为宾语从句,如：He wants to know whom/who he should ask about Father Christmas。He doesn't know what to do。=He doesn't know what he should to do。I don't know when to start。=I don't know when I should/will start。等等,很多。当主句的宾语和从句的宾语一致时,也就是同一个人时,可以互相改写。我的解释绝对正确,我是英语老师,如果还有不明白的请说。