Divorce is tough, and even more difficult when one person finds a new partner- whether a girlfriend/boyfriend or new mate. One of the difficult parts is that you have no control over what your ex-spouse does. If he chooses to introduce your son to his girlfriend, there's nothing you can do.
It's best to be supportive of each other, in front of your son. You may not agree that your ex exposes your son to his girlfriend (and possibly a new girlfriend next month) but you can tell your son that his Dad loves him, and you love him-life changes, but you will always be his mom, and his dad will always be his dad.
Kids are more flexible than we are as adults. Your son will reflect your attitude, so it's best to keep things positive.
Shirley Cress Dudley
Director of The Blended and Stepfamily Resource Center
Author of Blended Family Advice
I would say i depends on the age of the child/children. At five they might become attached or really resentful. Think about how your child is taking the divorce, how is he when he switches teachers at day care, etc. these type of questions might give you an idea of how he will take this. However a few weeks does seem a little quick.