有语法错误,what I don't want to say so much 是一个主语从句,he 也是一个主语,一个句子中有两个主语是不对的,a assiduous 应作 an assiduous.
看这个句子,不知道 what I don't want to say so much 是不是“不用我说太多”的意思,如果是,那么应在后面加上：is that,即：
First of all,what I don't want to say too much is that he is an assiduous student and has a good record.
以上就没有语法错误了,但要更地道的英语应再加修改：
First of all,he is,needless to say,an assiduous student with a good record.