T.S.
Sorry E.. That really sucks.
As a divorced mom myself, I totally know what it's like to feel like you can't keep your ex's decisions from impacting your child.
The only advice I can give you is to put your energy where you CAN actually do some good. Your ex is going to do what he does. It's going to seep into your son's life in whatever way it does. There's nothing you can do about that, so don't make yourself and your son crazy over it.
Instead, focus on ways to make his choices less damaging for your son. Think through what it is that is troubling about his dating choices. What is the risk? What are you worried it will do to your son? Then think of ways (unrelated to dad's relationships) to make sure that doesn't happen.
Also, try not to let it bother you (I hate when people say that) but your son is undoubtedly attached to you and could be feeding off of your stress and worry as much as anything else.
HTH,
T.