Right on the money SD I had 3 stepsons. and 1 of my own so we have went thru the gamut of BS with teenage hormones, their dad and mines mom for that matter undermining anything we tried to instill in them. The best thing is to be his constant go to as a male role model never try to be "his dad" but he has to respect you and if you and your finace' are on the same page with discipline that is the key, and consistency if it aint ok today tomorrow is gonna be worse punishment cuz it aint ok.
Wait til they get the p juice jesus it was like world war 3 over here, and 1 actually went to his daddy's cuz he didn't wanna follow the rules here. And that's ok cuz me and my old lady pay the fuckin mortgage not some 15 year old with a raging boner. So asta la vista baby. Its tougher than anything I have ever done in my life man. But most will eventually get it.
Consistency, same page (they love to use one against the other)and you have to also see from his little perspective. He was the "man" of the house before you were in the picture so he feels like he is losing his mother to you. You have to let him know that that will never happen. I had that problem with our oldest bc for a couple of years before I came along he was the only male in the household all the time, she had split custody with the other 2. And he was threatened by that, and acted out to try to run me off (he admitted last year in a Father's Day card). But I stayed consistent and persevered thru it and I know you can too.
NEVER say anything about how his dad is. Point out the bad behavior his dad exhibits and ask him to elaborate on it. but it should never come from you as criticizing his dad