He is her son,than whom a kinder son does not exist.

参考译文：他是她的儿子，她的儿子是最善良的。

参考译文：他是她的儿子，比她儿子更善良的不存在。

参考译文：他是她的儿子，没有比她儿子更善良的。

句子语法结构分析如下

主句主语：He（他）

主句系动词：is（是）

主句表语：her son（她的儿子）

引导非限制性定语关系代词：whom（whom指代前面名词her son，同时在定语从句充当介词"than"的宾语，与介词than一起构成介词短语又作后置定语修饰a son。)

修饰定语从句主语的前置定语：kinder（更善良的，更仁慈的，更体贴人，更好心的，更乐于助人的）

定语从句主语：a son（一个儿子）

修饰定语从句主语的后置定语：than whom（比她的儿子）

定语从句谓语：does not exist（不存在）

(特别备注：定语从句中，如果直接放在介词后作宾语，关系代词只可以是which或者whom, 指物/事，用关系代词which，指人用关系代词whom

(1)先行词+介词+which+主语+谓语+其它（备注：前面先行词属于主句某一成分。先行词为表示“物或者事”的名词）

Where is the supermarket in which you  bought the laptop last week?

Yesterday we visited  the West Lake for which Hangzhou is famous.

I have about 20  books, half of which were written by Bing Xin, the famous  writer.

The building of  which the roof was damaged in the earthquake has been  repaired.

=The building whose roof was damaged  in the earthquake has been repaired.

Can you tell me the reason for which you were late for  school?

=Can you tell me the  reason why you were late for school?

The naughty boy made  a hole in the wall through which he could see what is happening  outside.

(2)先行词+介词+whom +主语+谓语+其它（备注：前面先行词属于主句某一成分。先行词为表示“人的”名词）

Do you know the lady with whom our  manager is talking in the office?

The old woman with  whom you talked just now is a famous artist.

老师，这是什么定语从句？先行词是谁？帮我分析一下句子成分。谢谢!

解答如下：

同意刘老师解答观点

以上句子是一个由“关系代词whom”引导的非限制性定语从句，关系代词 whom指代的先行词为 “主句的表语成分”“ her son”(备注，“than+ whom” 属于介词than+关系代词whom结构，即whom=her son。关系代词whom在定语从句作介词than宾语）

上面句子可以和下面几种句子结构相互转换

He is her son, than whom a kinder son does not exist.

=He is her son and no one is kinder than him.

=He is her son and a son kinder than him  does not exist.

=He is her son. Nobody is kinder than him.

=He ,who is her son, is kinder than anyone else .

=He, her son, is kinder than any son( in the world).