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 is His slave and Messenger.
If the innovation of the Imam is one that takes a person out of the fold of Islam, such as him saying that the Qur'an is created, then the prayer is not valid behind him. In which case, you pray the Jumu’ah as Thuhr prayer [4 Rak’ahs] if you do not find a mosque where there is a valid Jumu’ah.
However, if his innovation is not that which takes the person out of the fold of Islam, then if you can find another Imam, then it is better not to follow him, but if you do not find another Imam, it is not permissible for you to abandon the Friday prayer [with this Imam].
The Fatwa of the Standing Committee for Issuing Fataawa reads:
“As regards the prayer behind innovators: if their innovation is that which takes the person out of the fold of Islam, such as calling upon other than Allah, and making vows to other than Allah, and believing in their Shaykh about something that is only peculiar to Allah, such as absolute knowledge, the knowledge of the unseen, or having an influence or effect on the universe, then the prayer behind them is not valid. However, if their innovation is not that which takes the person out of the fold of Islam, such as mentioning Thikr that is confirmed from the Prophet but they do that in congregation while leaning their bodies left and right, or front and back, then the prayer behind them is valid."
But a Muslim should pray behind an Imam who is not an innovator, so that his prayer will be greater in reward and he will be far from evil and falsehood.
Allah knows best.