I have not spent a full night away from my girls yet, even when I had DD2 I was there to put DD1 to bed and did not leave home till 1am. So I am not much help there.
I would be telling your DH and MIL that DS is not a share toy, he is your baby and when you feel ready for him to spend a night away then you will let them know. Until then don't keep asking because your not trying to be insulting, just trying to be a mother.