Being a stepparent is honestly one of the hardest jobs in the world and nobody really prepares you for it. Kids often see stepparents as threats or replacements for their real parent, and that resentment can come out as disrespect. But that doesn’t mean you have to just take it.
How old is he? Because the approach is totally different depending on whether he’s 8 or 18. With younger kids, you can build a relationship over time through consistency and patience. With teenagers, it’s harder because they’re already set in their ways and more likely to push back.
Your husband needs to be the one handling this. It’s his son and he needs to set the expectation that you will be respected in your own home. If he’s letting his kid treat you badly because he feels guilty about the divorce or whatever, that’s a problem with your marriage, not just with the kid. You and your husband need to be a team.
That said, try not to take it personally. This is probably more about his feelings about his parents’ split and his loyalty to his mom than it is about you. But understanding where it comes from doesn’t mean you have to accept disrespect. Set clear boundaries and let your husband enforce them. And give it time. Some stepkids come around eventually.