if you choose to act in a violent way towards the ex, you really wont be solving any matters. oh yes, it would be so nice to rip someone's head off and mount it over your fire place... but in reality, that's never the best solution.
until either one of those two decide to put an end to their drama, it's never going to get better. believe me, once someone has had enough, they will make it a point to be done.
i would just keep my cool sweet heart. the child needs a stable person in it's environment, and with you showing to your boyfriend that you can remain level headed when the ex's act outs flare up may actually help him to try to do it as well. lead by example. it may take time, but remind him by your actions why the fighting is unhealthy. feeding into the emotions will only post pone any healing for any of you. you got to be stronger and smarter... plain and simple...
as for custody arrangements, if you are willing let him know you want to be there for him and the child no matter what. show him if you are ready, that your are willing to become a third parent figure to the child. he may end up getting full custody, and this child will be there all of the time.
i know it's hard right now, and i know you feel like snapping. these feeling are totally normal and healthy. just don't create more drama than is all ready present. that wont do any of you any favors.
good luck,
~fawkse~