What possible effects will having a father who keeps possible mother-figures away from a son have on him?

Just try to keep things they way they are for a while, don't rock the boat by talking serious to him yet, if you think that might scare the dad away. You see that things are good for his son now. In time his dad will see it that way too. He just may have some mental block about "commitment". Since he is not into having one women them might back fire on you and the last thing the boy needs is another voice on the phone. A good relationship starts with best friends give it time. Time is the healer of everything.