All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu 'alayhi wa sallam, is His slave and Messenger.
It is correct that his righteous deeds will remain for him and will admit him into Paradise with Allah’s mercy. As for the highest level that he will attain, this belongs to the Unseen and we do not know any Hadith in this regard. However, a great Hadith that indicates Allah’s great graces was reported in this regard by Al-Bukhari and Muslim on the authority of `Abdullah ibn Mas'ud who said that the Messenger of Allah, sallallaahu 'alayhi wa sallam, said: “I know the last of the people of Hell who will be brought forth from it, and the last of the people of Paradise to be admitted to Paradise. (It is) a man who will emerge from Hell crawling, and it will be said to him: ‘Go and enter Paradise.’ He will come to it and it will be made to appear to him as if it is full.’ Allah will say: ‘Go and enter Paradise.’ He will come to it and it will appear to him as if it is full. So he will say: ‘O Lord, I found it full.’ Allah will say: ‘Go and enter Paradise.’ He will come to it and it will be made to appear to him as if it is full. So he will say: ‘O Lord, I found it full.’ Allah will say: ‘Go and enter Paradise, for you will have the like of the world and ten times more, or you will have ten times the like of the world.’ He will say: ‘Are You mocking me, or are You laughing at me, when You are the Sovereign?’” He said: “And I saw the Messenger of Allah, sallallaahu 'alayhi wa sallam, smiling so broadly that his molar teeth could be seen.” And he used to say: “This is the lowest of the people of Paradise in status.”
Allah knows best.