My daughter is 8 and her father and I have been separated since she was a few months old. However, we have worked EXTREMELY hard to maintain that the three of us are her family unit. We have weekly "family nights," where we go out to dinner and be together as the three of us. Her father and I also talk nearly every day about the day, about issues, whatever.
Our rule was always that our daughter would not even *meet* another individual until it was very serious, and then we would discuss it (her father and I) about how it was going to work, etc. before she met the other person.
In the last couple years, her father has begun dating a woman and it has become very serious. He waited over a year to introduce this woman and has moved really slowly with introductions, etc. Recently, we just began dealing with this issue of sleepovers. I am forcing this to move more slowly than he would like, but to me, it is VERY important that my daughter:
a) be very comfortable and feel very safe about the entire situation
b) have the most stable situation that she can be in
c) know for absolute certain that the 3 of us are still her family unit - and that this other woman is becoming part of our family, but not replacing it
d) learn about the importance of commitment and an appropriate speed of relationships
It took me a long time to be ok with him having sleep overs with this woman at his house - and I really do like, enjoy, and respect this woman. Recently, he planned on the two of them sleeping at her house, and I am not ok with that. It takes away from her time with her father and from her security about knowing where she's going to be.
My personal choice for myself is that I am not going to date until my daughter is older or potentially even out of the house. At least not visibly. My focus is keeping the two of us going and strong and making sure that she is happy, healthy, and learning the lessons about life that I want her to learn. My preference would be that she wait until she was at least in college to have sex. I also want her to see that a woman can be happy and successful without a relationship. At the same time, I am grateful that her father is in a committed, healthy, strong relationship that she can see and learn from how to make that work. I wish, however, that they were not sleeping in the same bed in the same house with her until they were either living together or married.
Does your son see his biological father? He already has two men in his life - to introduce a third too soon or men that pop in and out after only a year is going to be really hard on him. I would wait as long as you can and then go very slowly. Your son needs to be your priority. However, there is nothing stopping you from dating, etc. when your son isn't around.