Hello Fresia,
I know this situation, to varying degrees.
I too am living with my BPD ex, unfortunately for 3 years now while we are "split".
We also have children, ages 16 and 11 and it has been trying on all of us.
I know 3 years may sound absurd (I certainly think it is like a never-ending nightmare). The length of my painfully long co-habitation is due to a few factors; I have developed severe social anxiety and can barely leave my home. He suffered a "break-down" that involved being institutionalized temporarily and then was in and out of intense therapy, not working, for nearly a year. During that time I was in a total fog of shock and developed PTSD, followed by 10 months of him returning to work (and before I got things settled to leave and was caught up on past bills, etc.) he left work again for almost another year, causing greater financial hardships. He insists on the illusion still that he loves me and wishes to be with me, so he refuses to leave (though his words and actions are two entirely different things).
I own our home but lack the social ability to go out and get the help I need to remove him legally, and due to my circumstances he has made threats to have my children taken from me, and other dramatic hogwash. It's such a huge mess.
I have my own room, which helps
tremendously.
I do as much as I can to avoid him and maintain a good relationship with our children. In ways they understand better then a small child simply because they are older, but that doesn't make it any less painful for them and they sometimes show signs of stress from it.
Everyone handles things their own way of course, and after everything I have been through I have become a very mistrusting person. However I feel compelled to add that if my child started acting out aggressively toward me in that specific situation, I would think my ex might be manipulating the child to do it (saying things like "Mommy is bad/mean/making daddy hurt/making daddy leave, etc." behind my back). I am not saying that I believe that is happening to you but I would consider the possibility.
I think giving your child pleasant distractions, playdates and time away from the house, is very beneficial.
Also you said you are supporting your ex emotionally. I hope you feel that is balanced. It sounds like you are already under enough strain. I think withdrawing completely, or as much as possible without seeming to be ignoring him, is the best. The less you have to interact with him the better. Don't strike up conversations, don't seek him out for anything unless you have to discuss your splitting arrangements and even during those times I recommend you have an outside 3rd party included. Put decisions in writing, too, and both of you should sign them. Close any joint accounts, etc. Normal precautions to protect yourself.
In my own relationship I have bent over backwards so many times, that now supporting him in any way would be on my list of 'the last things to do, ever'. Not in a way of bitterness or out of anger, just because I am trying to focus on myself and heal, as I am so drained and have suffered so much.
I do encourage you to sleep elsewhere, the sofa if need be, or better yet perhaps you can spend some time with a relative or go to a hotel, just to give yourself a feeling of safety and allowing yourself to rest.
Also "our roles in the house more confused than ever" sounds like you need to set some boundaries.
As for wanting him to move out, most of what you can do is ask, and most of the time the answer is a big fat "NO". I do know that if you are not married you could go through normal means of an eviction notice, depending on details of if you rent or own, etc. However you said you are looking to leave, so perhaps just renting an apartment nearby as to not disturb your child's schooling for a while would be the easiest solution, while waiting for your house to sell.
The most sound thing I can tell you from my own experience is the sooner the better. Sticking around longer then intended has done a whole new level of damage.
Sorry, I ramble on
I know my situation is abnormal, due to my own developed problems. I did try to end the relationship and get him to leave but, unfortunately, he attempted suicide and I spent a long time after feeling trapped. I just try to serve as an example of what you do not want to become.
Run to your freedom! I know you are scared, and probably heartbroken and in great pain but the best thing you can do right now for yourself and your little boy is move forward as fast as you can.