Can it be a mistake? You should talk to him directly before assuming the worst

That's fucked up.

Well let's hope you had a prenup and he broke the infidelity clause and you get to keep your innocent 5 year old to yourself, cause at least they're an honest and sweet child of yours.

how did your five year old know before you do? you should try talking to your (ex?) husband to confirm it, first. if he is leaving you for someone else, well let him, but make the "new girl" is aware of it. he might do the same thing again.

Your soon-to-be ex is a coward. Even if it wasn't his plan to have your child tell you (and if it was, that's even worse), the fact they kid even knew before you is very telling of his inability to face you and do his own dirty work. That's beyond reprehensible.

Don't shoot the messenger

That act alone should show him as an unfit parent, and go a long way towards giving you primary custody, or maybe even sole custody. No child should ever be put in that kind of position. All of the ugliness of divorce should be kept away from the children, and both parents should keep a united front to them, letting them know that even though the parents couldn't work things out that they both love the children as much as ever.

That's disgusting and disrespectful for you and your child ... Having so little respect as to use one's child as a messenger for the sort of thing that you're supposed to be preserving them from ... Good luck OP, you're worth much better than him !

That's fucked up of him, to make your child tell you.

I'd bring this up in case of a custody battle.