I'm glad you have plans to go OP.
I think everyone calling the 12 year old a 'brat' is unfair.
This young man has had this behaviour modelled from day 1 by dad, and put up with by mum. It's not his fault this is his male role model.
The absolutely best thing you can do for this young man is to move out, establish a calm home with boundaries, hope he doesn't see his dad too much (but you cannot control that), get some solid consequences in place (so, lifts, internet, money are only given to teenager in my house who are reasonably pleasant and don't make my life a misery, a one-off teenage tantrum with an apology would be ok). I don't give a lift on the day or give money to teenagers who are rude or aggressive to me, full stop (as I say, if they burst into tears, or have difficulty regulating emotion, I do give a bit of leeway and suggest they go to their rooms, or accept an apology). Perhaps therapy might help.
There's a danger coming very fast here that what is just verbal abuse now might spin out into genuine contempt for and dislike of women, and make him think it's ok to treat a partner like this. Please act as soon as you can, as he sounds like he has some lovely qualities in there and in a different environment you will be able to pull those out.
Also, act before he is huge, and when you get into the new home, make sure it is clear you are in charge, not in a dominating way, just very calm and assertive- it is hard for single mums to deal with rude big adult sons, or indeed rude big adult daughters, I think you have to make sure they know in a very subtle but firm way that you pay for the home, your rules are in the home and they do not run things, otherwise it can spin out of your control. I love living with my teens by the way, and we have got that balance, but when there's one mum and difficult big stroppy teens it can lead to power issues that don't help them become their best selves either.
I wish you so much luck with this OP, as I think you know what he and you need.