All this person who mistreated his parents has to do now is hasten to repent for Allah has promised to accept the repentance of the one who repents. Regretting mistreating his parents is repentance in itself. After a parent's death, one can honour them by making a lot of du`a from them, doing righteous deeds on their behalf, and honouring their friends and loved ones.

If a disbeliever or apostate becomes Muslim but persists in committing a sin from which he has not repented, his becoming Muslim is valid according to scholarly consensus. But will he be forgiven for his sin from which he has not repented by virtue of becoming Muslim? Concerning that there are two scholarly views. See a detailed discussion of that in the long answer.