OMG!!!! I thought I was the only one.
Although the step-child in my life is a few years younger I have been dealing with a similar issue. Like your step-son, he ignores me when I talk to him, constantly reminds me that I am NOT his mother, make a mess just to spite me ... the list goes on. We both have adult children from previous marriages & they too see the issue I'm facing (this child is a result of his mid-life crisis gone wrong (heehee!!)).
When I have an issue & say something to his father his reply is always...."that's between you & him". Over the past few months I've come to realize that his father will be of no help and so I have begun to fire back; using his <the child's> words & gestures against him. When he asks me to help with or needs something I started ignoring him and occasionally I will remind him that I am NOT his mother. If he throws a fit, so do I. If he won't clean up after himself I leave the mess. He's now seing how it feels to be treated like a door mat and guess what...HE DOESN'T LIKE T!! So, things are getting better between us. One thing I've also started doing when he comes to stay with us is ask him to take of his mommy's hat & put on the daddy's hat; different house different rules. This seems to be working well as he is beginning to develop a level of respect for me & my home.
As far as the mother goes, I've had to remind her that deep down she knows i love her son as if her were my own & would never do anything to hurt him & while she may believe her child does not lie (yeah right!) he may not tell the entire truth & even exaggerate things and does so because he knows how she will react; this is my house & what goes on in it is none of your business...I don't but into the things that she allows him to do, or not do in her home & I even follow through with any type of punishment she's imposed to keep things as consistent as possible & I DEMAND the same respect. She's a very young mother of 5 (told you ~ mid life crisis gone horribly wrong) and is still too much into herself to see what's happening.
Well this is my story. The only advice I can give is if you loves this man you have to keep in mind he is a package deal and that you have to be patient with the both of them. But stand your ground and if you have to go on strike and let the house be messy do it. If his dad is willing to do his laundry, let him; eventually dad will get tired and so something about the situation. Talk with your kids and let them know why they have responsibilities and how this wil benefit them in the future. They see what's going on and while it may, at times, upset them & seem unfair they will soon come to understand.
Good Luck!!
GMGB!!