M.P.
K.:
You may not like my response, so read on only if you want an honest opinion.
Although your ex should be respectful to you for the sake of your son, it is not your decision who can and cannot be around him when he is with his father. Who your ex chooses to date is not your concern. However, if he puts your child in danger or around a potentially harmful situation or a dangerous person, you can request suspension of parental visits; but to just decide of your own accord that this person your ex decides to be with is not suitable to be around your son is ridiculous. You aren't helping the situation by trying to control the situation.
I am both an ex-wife and married to a man who has ex-wives, and I've been in this other woman's shoes, except I am not 16 years younger than my now-husband. Having been on the other side of this equation ( his ex-wife trying to control with whom her ex can have a relationship and who he can bring around her kids)I find it disturbing that you are assuming he should follow some rule you've decided should be set about dating. It is in the best interest of everyone if you just remind your ex to think of the well being of your son and to always keep your son's safety in mind and not demand that who he dates meets your approval.
If you were to put your feelings aside and make an effort to become friends with the other woman, she'd be much more apt to take excellent care of your son. You have the opportunity to set an example of how an adult woman should act for your son and your ex's new girlfriend.
If you respect yourself, and your son, don't act like the typical ex-wife - and, before anyone gets their tailfeathers ruffled, not all ex-wives are "typical!"
Either way, I wish you luck.