I heard he got boon that after this life he will live in dhruv loka as king.
I am unaware of the boon Lord Vishnu gives to Prahlada being the ruler of polestar. There were no rulers of the Polestar before Dhruva, son of Uttānapāda. But there are other boons mentioned in the Vamana Purana. He got immortality, everlasting youth but not as Dhruva.
Lord Vishnu gave boons to Prahlada as the incarnation of Narayana, a Sadhya and son of Lord Dharma. His brother is Nara. He is also an expansion of Lord Vishnu.
Prahalada once waged a war with Nara and Narayana. He was unable to defeat them (Narayana particularly) even after several thousand divine years. Feeling frustrated, he prays to Lord Vishnu and asks the reason why he was not able to conquer Sage Narayana. Lord Vishnu says that Narayana is none other than himself. He advises him to defeat Narayana with devotion. Knowing this, he prostrates to Narayana and praises Him. Then sage Narayana says to Prahlada.
Lord said - O great resolute! O Daitya! I am pleased with your prayer. O Daitya! This exclusive devotion has defeated me. [9.56]
O Daitya! The defeated man is given punishment but instead of receiving any punishment from you, it is I who will bless you. Ask for anything you desire.
Prahlada asks for the removal of his evil sin he committed by waging war with Him.
Prahlada said - You are competent to bless me with what I ask for. Hence O Jagannatha! May the evil committed on my part by body, mind and speech in course of fighting, all that may be absolved. I pray and ask for it. Please, bless me with this boon.
Narayana said - O demon king! May it as be you wish. May your evils so committed be absolved. O demon! ask for anything more. I will render that also.
He asks that all the thoughts in his mind should shelter on Lord Vishnu, for the worship and meditation on Him. Narayana grants that too.
Prahlada said - O Adhokshaja! I have got everything with your grace. May my popularity live long with your grace.
Narayana said - It shall be asked. Besides it, you shall be immortal, always youthful and invurnerable with my grace.
He advises Prahlada return to his abode and continue his righteous way of life and be the preceptor of the Daityas and Danavas. Prahlada takes leave from there and coronated Andhaka (son of Hiranyaksha) as the king of demons.
If Lord Vishnu really said same then does that mean Prahlad(as dhruv) will still be stay safe even after mahaa pralaya at the end of Kalpa?
He attained the state of immortality by absorbing his interest on Lord Vishnu. He attained the purity of body through Yoga. The chapter doesn't mention him to be as a ruler of North star.
But, Dhruva will remain safe even after the end of the Kalpa. Lord Vishnu gave this boon to Dhruva, son of Uttānapāda. He is the son of Śatarūpā and Svāyambhuva Manu. From Bhagavatam,
The Supreme Personality of Godhead continued: My dear Dhruva, I shall award you the glowing planet known as the polestar, which will continue to exist even after the dissolution at the end of the millennium. No one has ever ruled this planet, which is surrounded by all the solar systems, planets and stars. All the luminaries in the sky circumambulate this planet, just as bulls tread around a central pole for the purpose of crushing grains. Keeping the polestar to their right, all the stars inhabited by the great sages like Dharma, Agni, Kaśyapa and Śukra circumambulate this planet, which continues to exist even after the dissolution of all others. [SB 4.9.20-21]
If we look at the text in bold, we will know that there were none who ruled the polestar before Dhruva, son of Uttānapāda. So, it is clear that Prahlada was never a king of the polestar.
The Supreme Personality of Godhead said: My dear son Prahlāda, all good fortune unto you. For the time being, please go to the place known as Sutala and there enjoy happiness with your grandson and your other relatives and friends. [SB 8.23.9]
So, it is clear that he is still in Sutala and not in the Dhruva Kshiti (polestar).
We know from above verses of Bhagavatam that Dhruva will stay after the end of Kalpa and Prahlada is in the Sutala loka along with his grandson, friends and relatives so, it is likely that he is still the ruler of the polestar. It also says that there is no dissolution of this polestar which continues to exist even after the dissolution of all the saptarishis. So, Dhruva stays in his loka even after the end of Kalpa. Prahlada also stays due to his boon from Narayana but not as the ruler of polestar.