This is Mr.John for whose son I bought a book yeasterday.请帮忙

因为人家想说的是谁的儿子,我是给他儿子买的书。son是名词,能加名词的关系代词只有whose。for来源于buy的用法,buy sth for sb。给某人买某物。for是介词,sb是宾语,在这里也就是他的儿子。介宾结构,whose是关系代词,就是关系代词做介词的宾语了。