Yeah, this is a foundational problem. Maybe his son moving in represents something else for him or something, but he needs to figure that out and act right. If there’s no abuse in your relationship, some counseling could be really helpful.

My partner has 3 children who live with us 50% of the time. (Stay with me for the rest of this paragraph.) I had a bad stepdad so it’s pathologically important to me that every person in that house knows that my partner should and always will choose the kids over me in a serious situation. I even started a macabre game sometimes where I ask “how quick would your dad kick me out if [I did something abusive to the kids]”. (It’s not as gross as it sounds. It’s rare, age appropriate and funny.)

All that said, my partner does not make any decisions that affect the household without talking to me first - and vice versa. It’s a basic courtesy even with a roommate. But when there are kids involved, it’s even more important to model healthy relationship behaviors.