If this is for real, what are you still doing there? Take your son and move the hell out.

She's broken all of the basic rules of being a stepparent. Quite frankly speaking also being a decent human being.

Do your son a favor and leave your wife and file for divorce. If you stay with her, he might blame you for not protecting him from your wife. 

Either your wife is abusive and you are knowingly putting your son in done, or you know your son is out of control and you won't do anything to correct him.

Either way, YOU'RE the parent. YOU'RE job is to protect and raise your son. His ADHD is going to require a LOT of work on YOUR part to bring under control. Not your wife's doing; YOURS.

Pick what you're going to do and do it - she either is abusive and you need to leave, or your son is a hellion who needs to be corrected and disciplined.

Sheesh, I can only imagine how bad your first wife is as a mother if your current wife, who is mean to your son, tells him he's stupid and not good enough and that his mother is a piece of sh*t, is an upgrade.  It is not OK for your wife to emotionally abuse your son.  He may be difficult, but he's 9 years old.  He's a child.  It is your job to protect him.  He's only going to become more difficult if he's treated so poorly.  If you want to stay in your marriage, you and your wife should really get some kind of family counseling  and parenting help and you need to make clear to your wife that emotional abuse of your son will not be tolerated.  How essential is it that you be out of town 2 nights a week?  Expecting your wife to be fully responsible for a difficult child while you're out of town is a lot.  

Expecting your wife to care for your child with ADHD really is expecting a lot. As for the rest...my stepmom told me I was stupid and verbally/emotionally abused me (as well as inflicted very strict punishments on me specifically to get me out of the way) when I was young, and I could never understand why my dad allowed it. I think 2 things are going on:

1) You are expecting too much (you just said above how you don’t expect your wife to parent your son, but she is with him 2 days a week, so you do expect her to parent him). She is resentful. I’m guessing she feels you aren’t strict enough with your son, so she has to step in and discipline to keep things reigned in for when she has him 2 days alone. If you are really honest with yourself, do you have guilt that your son is a child of divorce? Do you let him get away with things because you don’t want to be “hard on him”? If so, that is EXTREMELY difficult and frustrating to deal with as a SM. Many of us are guilty of pushing our DH’s to discipline more, when our DH’s act like a Disney dad. If she is criticizing your parenting, you may need to honestly look at if you are really parenting, or just kind of letting him get away with things.

2) The above reasons may be why she is acting how she is, to an extent. As for calling him names- inexcusable. 

I would recommend sitting her down and asking why she she feels it’s ok to call your son names. Because it’s not, ever. Also ask what frustrates her about your current situation with your son, and really listen to what she says. If you want to stay in this marriage and improve this situation, I’d bet you have some work to do in the parenting dept, and figure out an alternative to her watching him 2 days. Either way, fix your contributions to this problem (yes you are contributing), and make sure the name calling stops NOW. 

Really? Your wife is abusing your son and you don't know what to do?

Take both kids and leave the abusive cow.

His wife abusing his son.  WHY doesn’t he get a normal job and is home after school and not away two nights a week to take care of his son ?  He dumps this kid in his new DW, who has ADHD, who also has a 4 yo, bio mother out of the picture., and wonders why DW is angry!  If he leaves, it would be a plus for current wife.  Stop complaining!  

Nobody feel sorry for you, Take your kid, and stop letting other people solve your problems, and then complaining about it. It’s your fault for having a kid with a Bio mom who doesn’t give a sh*t, who you most likely dump kid on her.  And live your life with out everybody doing your job 

my current wife will speak badly about my sons mom straight to his face, and tell him his mom is a pice of shit and dose nothing for him

AND

I have witnessed on many of occasions her mentally abusing my son. Calling him stupid and not good enough. She is constantly yelling at him

+++++++++++++++++++++++++++++++++++++++++

No way….

Holy crapola…ANY person that talked to my kid like that, would be seeing a side of me they would NEVER EVER forget.  I just can’t imagine that you are ok with this.  And you must be ok since you are still there.  Do NOT let her watch your kid EVER again.  Do NOT ever let her talk to your child like that again.  I don’t give a crap how bad your kid is…he does not need the abuse.  Get some counselling or get out. 

So leave. If she's really as awful as you say. It sounds abusive. However, we all know people tend to notice things or exagerate memories, it's how people are wired. So really evaluate, if it's true, get your poor kid out of there. If not, discuss the frustration with your wife and figure out what the heck is going on. Then possibly still leave, or get a different job with better hours, or get after school care for your kid. Idk, just SOMETHING. But this is an action type of situation.