I think you have to be careful in this situation. Think carefully and make sure the wording reflects your exact desire. You might be better off by using a different kind of spell - one that promotes a more positive interaction between you all. Think about creating a transformation spell that takes your current relationship with the father and your son, and turns it into a supportive, friendly one.
Also, I recommend you experiment by changing the way you deal with your ex. Don't assume a negative position, instead visualize a friendship developing between you. Smile and be more ready to agree to arrangements. Don't set yourself up as the enemy. When he acts up, let it go and instead try to be pleasant even if he isn't. At least you'll know that you have the higher ground and it might give him something to think about.