A.S.
I am a single mom as well and I wish you guys the best of luck in dealing with these hard issues. Remember why things didn't work out? You know it was for the best even if it is hard on your sons and you some times. It would be even worse for your son to see two adults he loves very much miserable with each other only tolerating there life together so that he can have two parents, he might just learn love doesn't matter or treat woman or people in general with out compassion. But if you show him love and explain what is going on in a manner that is civil, He will see you as a strong brave mom who has his best interest and feel safe and secure. (I am so hoping this is the way it goes, this is my motto)
I have the same problem, my daughters father (doesn't see her much) has a new live in girl friend one right after the other, he is incapable of being alone and wont leave a relationship even if it is bad. She is three now but she will grow and I hope to show her how a true relationship works.
The thing is "you are going to screw up your kids, just try to minimize it" There really isn't away around it just set a good example. A war will only hurt the babe.
I wish I had more answers for you.
(I hope this came out like I had hoped)