I can picture it Liz. I don't feel comfortable with other people's kids being over-indulged (in my eyes) and I don't live with it every day. That's just my example, not necessarily yours.
OTOH my son lived with his s-mother and father when he'd just left school and started his apprenticeship. I am not too focused on housework, she is v v fussy, so there was a lot of conflict. That plus my son resenting this other woman with his father. I didn't interfere, as I could see her POV as well as his.
And my son's father really did have a wicked s-mother who caused real harm to him and was fairly open about being jealous of him. We all felt disappointed that his father was too weak to stop it.
I can see that if you stay the course your problems will gradually reduce as the children become more mature, like your son. All I can suggest is keep an open mind, keep talking to your BF so things don't build up, and possibly start a family meeting, say monthly.
I'm sure you're handling it as gracefully as you can, and know that young people can be painful, whether step or your own.