What is the correct Islamic stance towards a Muslim who practices sodomy and sometimes leads people in prayer in the mosque, and how should he be dealt with in terms of boycotting, advising, or concealing his sin, and what is the ruling on praying behind him?

It is not permissible to accuse a Muslim of indecency without clear proof. If sodomy is proven, which is one of the gravest sins, then it is obligatory to advise the perpetrator and warn him. If he does not heed the advice, then it is permissible to abandon and boycott him. The general principle is to conceal a Muslim's faults, but if concealing them leads him to persist in his wrongdoing, then one should inform someone who is able to stop him. Reporting him without a valid purpose is not permissible. Praying behind him is permissible if his prayer is valid, though it is preferable for the Imam to be righteous.