I understand and don't think you're overreacting or being a control freak. Obviously this needs to be dealt with quickly and quietly for the sake of all involved. You and your ex, need to be open and honest with each other, because ultimately both of you want whats best for the child, and that includes, niether of you being overly worried while he's with the other, and all getting along as well as possible.
I think you need to calmly have a chat with your ex, without your son present. Explain to him, that while you are happy that he has moved on and is seeing someone, you would like to have been told about his new relationship and would like to have met the woman in question, before your son stayed over. Just so as you know who is around your son. Tell him that you understand that he would want to be kept informed of any man you had staying over, but that needs to work both ways.
Obviously it is too late to meet her before your son now, but perhaps you could all meet, to show your son that there are no hard feelings and no secrets!
Good luck, I hope you manage to get this all sorted out!
.....and for what its worth I hope that your ex is serious about this new woman because IMO, I dont think either of you should bring new parters into your sons life, untill you really know them, and its a serious relationship.