Hello, and thanks for requesting me to help with your question.
I completely agree with you that the eldest son is showing you disrespect. He is at a difficult age to begin with, but this doesn't excuse his behavior. He most likely resents that his parents aren't together and therefore, resents/blames you for being with his father. When in your home, certain rules need to be obeyed and this is for you to speak to him about, but, more importantly, it is his father's job, since you are not his mother, to speak to him about the way he interacts with you.
Speak to your boyfriend, his father, and let him know what's going on, if he is not aware of it. He needs to have a discussion with his son and let him know that he is not behaving appropriately in your home. It is not only your home, if you and your boyfriend are now living together--it's your 'family' home and there are rules.
If his father doesn't know about this behavior (he may only say things to you in private, when his father isn't around), of course, let him know and discuss it, to come up with a solution together. If he does know and does nothing, he needs to be come a stronger parent.
At the age of 14, there are many privileges that can be taken away from the boy, in this electronic age. He most likely has a cell phone, computer or tablet, and watches TV. If he is disrespectful to you, his father can take away his cell phone for a week, or not allow him to use the computer except for schoolwork, or take away his TV privileges for a certain period of time. He can be 'grounded' and not allowed to get together with friends after school or on weekends for a week. The boy's mother will need to be cooperative with this and help enforce the punishment, if the son is living with her during the week. She might be bitter that you are her ex-husband's girlfriend and not want to cooperate, but that would make her a parent that is not interested in the best interests of her son and teaching him the 'rules of life:' you do something inappropriate and there is a consequence.
It also might be a good idea to attend family counseling with an experienced therapist and all three of you (you, your boyfriend and the 14 year old son) can discuss this in a 'safe' environment with an unbiased professional. Perhaps s/he can find out the cause of his behavior toward you or at least get him to admit it. In the meantime you can tell the son, every time he disrespects you that it's inappropriate, and he will lose a privilege for his unacceptable behavior. You can explain that you are not trying to take the place of his mother, because you know he already has a mother, however, 'this' (yours and your boyfriend's home) is also YOUR home (you're speaking to the boy, now), when you come to spend time, and all family members are expected to show respect to one another.
The key here, is to involve his father and have him have a heart to heart talk with his son about the behavior that is expected in your home and always, in respecting his elders.
In addition to consequences for inappropriate behavior, there can also be a reward system for good behavior. You can plan trips to go have ice-cream, go to a park, a video arcade, etc., only if both boys behave well.
I hope this helped and things improve soon!
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