Just that he was sitting in/on another/a different chair. As it's 'ann an cathair' the chair is probably an armchair. See the discussion here re in/on and chairs: https://forum.duolingo.com/comment/36179930/C%C3%A0ite-a-bheil-mi-a-suidhe-Ann-an-cathair
esan is the emphatic form of e - Gaelic uses a different form of the pronoun to show emphasis where English relies on a change of tone of voice. "Bha e ann an cathair eile" = "He was in another chair", while "Bha esan ann an cathair eile" = "He was in another chair". The same goes for all the non-emphatic/emphatic pronouns: mi/mise, thu/thusa, e/esan, i/ise, sinn/sinne, sibh/sibhse, iad/iadsan.