I don't know the reason why he was absent yesterday. reason后面是宾语从句么？

是定语从句
reason 是先行词
why he was absent yesterday是用来对reason 解释说明。

不是宾语从句，是原因状语从句
这句话的意思是我不知道他昨天为什么缺席。

定语从句，修饰reason

定语从句,why he was absent yesterday用来修饰reason表示...的理由