I too have 12 yr old step son with ADHD. Lying is not part of ADHD. Lying is a learned behavior or a way to manipulate a situation. Medication can help with ADHD but not lying.

Keep recording conversations.  It will soon be realized he is a lil liar liar pants on fire. Protect yourself and your child. 

Have you played the recordings for mom to hear that her precious is a liar? If she's still believing the child over her spouse there is a serious problem. Especially if it's recorded. You can't debate or rationalize with that kind of crazy. If that's the case, GET OUT as fast as you can. Mom sounds like she might have a screw loose and it will just get worse. If she is taking the frustrations out on an innocent child (yours) with her finger pointing, your job as a parent is to protect your son. 

I don't know exactly what is going on in your household but it sounds toxic.  It's up to you to decide if it is. If it's toxic, if she is not thinking and behaving like a sane person protect yourself and your son by whatever you need to do. If its move out, do it.

What does extreme adhd look like?

Is your bio son of this marriage OR a prior marriage. Hopefully prior marriage. In any event since you have custody of him (assuming) you duty is to protect him. IF bio dad gets wind of this he could file modification for change of custody. Just letting you know.

If you are unable to afford housing, you may qualify for housing assistance with Social Services. Doesnt hurt to find out. Also food stamps and maybe help with other areas.

Sorry to hear about your situation.  My SS had ADHD and still does at 23.  It was  a tough time and will always be.  He would get invited over to people's houses once for a sleepover or to play and then never get invited back.  The lying and manipulation will get worse as will everything compound when he gets older.  The other issues you describe are not ADHD.  I'm not an expert but it sounds autistic or something to that effect.  Good luck.

Honestly, they were awful and worse.  The arguing and defiance, you name it..  I would not do it over again.  If we didn't have a daughter together I would have left.  We were supportive of each other though and on the same page.  There was never any question of who my wife believed as that was not an option. 

Put yourself in the shoes of your bio son, and do what's best for him. He'll only be a kid for a few years, and deserves to live in a peaceful home.

 Individuals who grow up with chaos and upheaval in the home often go on to perpetuate that in their adult relationships. If you want better for your son, find other accommodations. You could try LAT (living apart, together) while continuing as a couple, just dating, or break up with your SO. But you have a responsibility to ensure your son is in a healthy environment, so please do so.

I agree with the post above that lying is not an uncontrollable part of ADHD. As someone with a DH with ADHD and a SS with ADHD, the lying is a coping mechanism to cover up for the ADHD - that is, the lying is used to hide mistakes made due to ADHD (e.g. forgetfulness, impulsiveness, etc). It is a learned behavior and it can be unlearned. It's also a common behavior for many people who have not been taught that lying is not ok. People with ADHD - even children - can control their lying and they even - as my DH always points out to his son - have a choice about their behavior and can control that if they are given the right tools to do so (including the correct medication and other tools and strategies for managing their ADHD). 

It sounds like your problem has more to do with the fact that BM believes his lies and he is using that to manipulate the situation. My SS who has been diagnosed with ADHD (I am skeptical of his diagnosis based on his behavior and what I know about his diagnosis, but that's another story), lies all the time to stir up drama and detract attention any time he does something wrong. BM believes him and tries to stir up drama with DH. DH typically ignores BM. BM herself also lies, so SS's lying behavior stems more from learning manipulation from BM than from ADHD (if he does have it), which would be from DH. 

My DH doesn't outright lie, but he fibs when he's forgotten to do something or when he feels like he should be able to remember something but can't (to cover up his shame at not being able to remember). I always call him on it and through that, he's been able to learn to admit when he's forgotten. Whereas, when he was married to BM, she was manipulative, so their marriage culture was just built on a web of lies and there was no "learning" that lies will only get you called out for not being truthful.  

My son is an Aspie. I believe she may want to get that tested - it's her kid.

That said, your sweetie taking her kid's word over yours is a huge red flag. MASSIVE, car dealership-sized red flag. It's also indicative of where the problem is. Her kid is a lying scheming PITA, she knows it and doesn't want to do anything about it, so she just shoves it all off on you by not believing you.

^ Reread that ^

She's in denial because she thinks it may be her fault. He knows he can squeak by, lying about little stuff, because she feels guilty he's that way because of her, and he knows he can get what he wants.

Not that it matters a lot. I'd be looking for the exit door.

Web cams and recorders to out the lies of the spawn on screen and on speaker ...... better yet..... call the locksmith, rekey the locks and let her know as she and the toxic spawn are leaving that equity life partners put each other first and that for sure includes trusting the partner over a lying little shit of a kid.

Good luck.