定语从句.但是关系代词在从句中作为was 的表语时,不能用which只能用that

首先,你举的例句语法是错的.whom是宾格,只能用来在从句中作宾语,不能作从句的表语.从语法上讲,应该用who：He is not the man who he was
这句改过后从语法上来说是对的,但是英语里从来不这么说话.为什么呢?因为who特指一个人,所以这么用有“他不是他”这样诡异的含义.正确的句子应该用that,即
He is not the man that he was
用that而不能用which,是因为从句是限制性定语从句（没有从句的话,主句意思就不对了）.
其次,不知是谁和你说“关系代词在从句中作为was 的表语时,不能用which只能用that.”这个说法不正确.用which或that,只取决于从句是限制性（that）或非限制性（which）从句.举两个个例子：
China is not the country (that) it was （限制性定语从句）
They told me that driving in that weather would be a bad idea, which it was （非限制性定语从句）

应该把whom改成who吧