Allah has not commanded the person who repents to do Wudu or Ghusl, whether his impurity is minor or major, either before or after repenting, unless he is repenting from disbelief or apostasy.

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.