K.N.
I know it sounds harsh and unfair to your son but being a parent in a very blended household the only advice I can give you is stick to your guns. If he shows up and says hey I want to see my son today and you have plans politely tell him that because he didn't call in advance you already ahve plans and he can't see him, if he's any kind of father at all it'll teach him to call. The same thing goes for if he is telling your sonhe will see him on said day and then him not showing up obviously this is upsetting to your little boy so calmly explain that if he tells you son he wil be tehre then he will be tehre or he won't get to see him. After all your son is what is important here. good luck.