Tell him off! Do you tell him off? Ignoring him is great, I also don't bite over stupid & small things but if that's not working & your DH won't step up, tell SS yourself to stop it & go away. I had to tell my SS's to 'mind their business' constantly when they were younger & always asking 'what are you talking about' it drove me nuts but they thankfully grew out of it.

When he interrupts, smile benevolently, use a kind voice, and say, "the grown ups are talking, wait your turn. " as if he were a not very bright toddler. Put him back in his place as a child without arguing or correcting. Arguing makes him think he's right so take that away from him. 

My ss14 and sd12 are both like this but it's over stupid crap. I ask if they are good on underwear or deodorant. Yes.....no they're not. Do you want to grab a sweatshirt for school Bc it's cool out?  Nope. It's everything and they always say no. But then they'll turn around and they text their das that they need underwear or deodorant or sd12 needs pads. I've quit asking them. I've been raising them for two years while we had full custody and bm got to pick and choose when she wanted them. I put my foot down. I wanted a schedule. I was done driving them to school everyday to a school outside of our district   They can rearrange their schedule because I'm not doing it anymore. I don't even really speak to them and do my own thing. My son is 18 and away at college. I'm done parenting these ungrateful rude kids. Their mom and dad can figure it out. 

My SS would do this to me as well, but probably not at the frequency you are experiencing. Every time he does this I would ask him if I was acutely talking to him to start with, if not, he can get lost. If my Dh and I are having a passionate discussion (aka argument), no kids are allowed to be present, whatever they need to say, they can freaking wait (in their rooms)

I do absolutely hate SS14's mansplaining though. He knows so little of the things but somehow is very confident of everything :/

SD used to asked a question that she knew the answer to. I would answer. Then should would agrue "Well I thought..." 

I finally told her to stop asking questions that she apparently knows all of the answers to. Sounds like your SS is a know-it-all too. 

Time to shut thislittle bastard down hard and constantly when he pulls this shit.  He gets zero say in any conversation between you and your mate. PERIOD!  He immediatley gets told to STFU and go to his room since it is an adult conversation and he is not an adult. Lather..... rinse..... repeat.

If he argues with your about anything, send him to do a research paper on the topic so you do not have to deal with him.  "Since you think  you know everything about it, you will go research the topic and give me a minimum of a 10,000 word paper, either in APA or MLA format, fully referenced, etc.... typed in double spaced entirely gramatically correct format, indented, punctuated, etc, etc, etc...."

Make sure he knows that you will  have the paper reviewed to ensure he wrote it and did not just download it from the internet. he gets to make zero comments on anything until he finished the paper as assighed.  Once he turns in a safifactory product, he gets to comment on that topic and that topic only.

Every argument, he gets to write a 10K word paper on the topic. Over, and over, and over again until he learns to STFU entirrely or speak intelligently and respectfully when in a discussion with you.

My Skid hated this and learned to approach his mom and I respectfully and after learning a bit about the topic he wanted to argue about.    We had some great discussions and debates once we implemented this model.

It was an extension of the 10,000 sentences model we used when he was younger. It was a great model to get him to think, follow through, and perform to his abilities.

Good luck, and enjoy the papers.

My step son is a knowit all. he relaly thinks hes so freaking smart and hes not !!!! i say black he says white, i say down he says up, he thinks hes soi smart it drives me nuts