I want to know if this hadith is correct. Some one told me that if one boy of the family memorize the Qur'an by heart which is "hafiz" that he will take 7 generations of his family into the Janna and if he doesn't follow the Qur'an then he will take them into the hell. Please explain me in details.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

According to our knowledge, we state that all narrations of Hadith concerning intercession of Hafiz to his family or so and so of them are not correct, i.e. weak and objectionable. Among these Hadith are:
"Whoever recites and memorizes al-Qur'an, follows its orders in Halal and Haram matters, Allah will enter him Paradise and will intercede for twenty of his family, whose fate is to enter the Hell-fire." Reported al-Tirmizi who said: 'It is strange Hadith with an incorrect chain of narrators.'
Another similar narration by Ibn Majah is very weak according to al-Albani (May Allah's Mercy be upon him).
A third narration reads: "whoever learns and memorizes al-Qur'an, Allah will enter him Paradise and grant him to intercede for ten of his household, whose fate is to go to the Hell-fire." It is an objectionable Hadith according to al-Kahteeb .
As for your question concerning the punishment of a memorizer of al-Qur'an who does not put its instructions into practice and that he will take seven generations with him to the Hell-Fire, this is a false statement which we should not give it a bit of attention nor should we ascribe it to The Prophet (Sallallahu Alaihi wa Sallam).
Allah says: {…..and no bearer of burdens shall bear the burden of another…… } [6:164]
{…….Every person is a pledge for that which he has earned.} [52:21]
Allah knows best.