Son's father asking for overnight privileges?

I think you are thinking perfectly correct here. You're doing the right thing. Besides, 18 months is a little small for sleepovers I think, but the other stuff would prevent me from allowing sleepovers even at an older age. It's probably his mother who is pushing for the sleepovers. I believe you should stick to your instincts. You sound like a good mother to me. Good luck.