What is required of the lisping person who began leading the prayer aloud as an Imam and then remembered during it that his imamate is invalid, and what are the consequences for those praying behind him?

The prayer of a person with a speech impediment is valid if he is unable to pronounce the letter "R" correctly. His prayer is not invalidated if he is followed in prayer by someone whose prayer is not valid behind a person with a speech impediment, for the prayer of the Imam is not invalidated by the invalidation of the prayers of the congregants. Moreover, one cannot definitively assert the invalidity of the congregants' prayer behind a person with a speech impediment, as some scholars deem it valid.