ما حكم الصلاة خلف من يقول بأن الله في كل مكان؟

If the speaker, by saying that Allah is everywhere, intends the belief of the Huloolis (incarnationists) that the essence of Allah is present in every place, then this is disbelief (kufr), and it is not permissible to pray behind him. However, if he intends that Allah's knowledge encompasses every place, then there is no objection to praying behind him from this perspective.