Parenting advice

My fiancé’s 5 y/o boy (we’ll call him R) will not respect or listen to me. He lies, hollers, walks away when I’m talking. My fiancé talked to his ex wife this morning and we now know that R has been lying to all of us about things while he’s at her house or ours. He told us this morning that since I’m not his mom, he won’t respect or listen to me. Other than that he says he doesn’t know what to say or I don’t know or because. Spankings and time outs are not working, it is wearing us down. We don’t know what to do.