Ahsanallahu Ilayk Theres a man at the masjid that consistently fills in as the imam when the main imam is not here. However, when the imam is here he uses a chair to do sujud and the sitting for fardh salah, even though he does not use a chair when he leads. Is it permissible to pray behind him?

Yes, it is valid to pray behind him, because when he leads he stands and performs the bowing and the prostration, so at that moment he is a complete imam and there is no issue. The scholars give the rule that whoever's own prayer is valid may lead, and the majority hold that even praying behind one who cannot prostrate is valid, with each person completing his own bowing and prostration. His using a chair when he prays on his own concerns his personal prayer, and if he is truly able to stand he should pray standing, but that does not affect the soundness of the congregation praying behind him.