Thank you for the information. It helps.
It's ok to have those feelings. It is natural to not want to move on from the comfortable and mutually satisfying relationship you had with your ex and your son. You became a family unit of sorts and to have someone you do not know intrude upon that can make you feel frustrated and upset.
You need time to adjust. This is the final step in letting go of the previous relationship you had with your ex. It's the hardest part, realizing that you need to move on. Part of your breakup was the idea that either of you could move on at any time with another partner. Having your son exposed to that is part of the break up as well, as hard as that is.
What you can do is talk with your ex, if you have that kind of relationship (it sounds like you might). Tell him that although you appreciate his need to move on, you are concerned with how it will affect your son. Make the focus on your son so you do not communicate your unhappiness with the situation. Your ex needs to know you are not upset about his new relationship, but instead about how your son will perceive this new person.
Set some ground rules with your ex. Since this is just a new girlfriend, she should not have any say so in your son's life. She cannot discipline him or direct him in any way, except in cases of safety issues (like he is about to touch something dangerous). Make it clear what your son is to call this person (by her first name with a miss or ms in front of it keeps it formal yet friendly). If the relationship should become more serious, then you can set new rules to deal with this.
Make sure you talk with your son during the time he is there, if he can talk on the phone. That way, you have a chance to make the connection with him and it will help you to know he is ok.
If you feel you are having too much trouble dealing with the situation after a few months, seek out a therapist to talk to. A therapist can help you work out your feelings and find ways to cope better. You can find a therapist through your doctor or if you attend church, speak with your pastor. If you want, you can search on line at http://therapists.psychologytoday.com/rms/.
Here is a book that can help you as well:
Sharing the Children: How to Resolve Custody Problems and Get on with Your Life by Robert E. Adler
You can find this book on Amazon.com or your local library may have it for you.
I hope this has helped you,