Keep trying! Do not give up. If you married his dad, for better or worse, than keep at it!
Being a stepmom is hard.........no doubts about it! I have been a stepmom for 17yrs.........the beginning was the most difficult (although until they were 18 it was ALL difficult at times). Let me tell you a few things I learned about this whole thing in my early years, to see if you can "relate". Perhaps your solution is in your "approach" to your stepson.
I learned that no matter how hard I tried, it was hard for my stepkids (then age 7 & 9) to allow me to be close to them. After years of struggling trying to form a relationship, I learned that the kids were simply afraid of hurting their moms' feelings, and didn't want to get close to me out of respect for her. NOBODY put this idea in their heads (although I wouldn't put it past their mother lol)......it was natural for them to feel this way. It took about 2 years to get through this toughest part.
How I got through it........I continued to beat myself up lol. Meaning, I didn't stop trying to play games, taking them to ice cream after school 1 day of the week, and making their favorite meals. I also showed interested in their bedrooms......always offering to make it better and more comfortable for them. In my case, they moved 3 different times (within a year) from the original house they grew up in, so I was sensitive to trying to make them comfortable in yet another "home". While doing all of this, it was most difficult to be "myself" cause I was trying so hard to please. But in the end, being myself, was what WON the battle.
Be your stepson's friend......be careful on the things you say to him to not sound like a "mother"........that can come later, but first he needs to learn it's OK to love you.......that is only something he can learn on his own, and only at his own pace.
Never, no matter what, ever bad-mouth his mom (or his mom's house, or his stepdad, the rules over there, or anything!) If you do, you will NEVER get through to him. And if you don't, he will remember and love & respect you even more later in his life.
Is it OK to have rules and chores in your home? YES! Just discuss it with him first......in my home, I pulled "homework is most important in this home, then responsibilities come next".........we only initiated 1 or 2 chores a week for a long time until routines were established/mastered. Then we added or adjusted from there..........but just because he has 10 chores at one home, doesn't mean he do the same in both. You need to take the pressure off of him, which will release negativity towards your home, but letting him get away without simple chores will backfire later.......so just a few is a good balance to work with.
I'm not sure of the details in your situation.......I would love to help you more. When you love someone who has children, it is sensitive and difficult to get through. After 17yrs, I feel like a "master' at it..........and the reward?.......I have a great relationship with my stepkids now........because I was always honest and myself........they come to me first for serious events/problems in their life knowing that they will always get the truth from me.......boy, it was all worth it!
Good luck! and keep at it!
~N. :O)