ODD Adult child moving back home

Hello Nicholeen! I just recently discovered your amazing videos and I am so grateful! I feel hope now, although it's still very new. I also just purchased your book bundle and my husband and I are reading together so we can use the skills with our 9 year old daughter. But my question today is actually about my adult 21 year old son that has moved back in. He's always been very oppositionally defiant and it's been extremely difficult and painful for me with my very sensitive personality. My son is very dysfunctional for various reasons, including a divorce and then the later death of his father. Recently we let him move back in because he was going to be homeless. The huge struggle I have now is dealing with his constant scrutiny about how I parent my daughter and his strong opinions. I've tried to ask him to let me be the parent but he still interferes. Some times his ideas are helpful, but most times I feel very belittled and scared. How would you recommend I deal with this in an appropriate way without making him feel unwanted? It's so hard because it feels like I'm walking on eggshells all the time.