Canonically, the last time we saw him was Empire Strikes Back. After that, however, we never really get to KNOW what he does afterward in the new canon.
Also yes I know Bossk made a small cameo in Return of the Jedi but he was in there for like 2 scenes in the background. If Disney is really gonna say Bossk died during the Carkoon Battle then that'll be more than disappointing for such a visually intriguing character.
At least Bossk is gonna appear in that new "Bounty Hunters" comic which is coming in March. But I'm 99.99% sure that takes place before Return of the Jedi.