That's a tough situation krazixs and in a way I'm in the same spot as my kids also spend time with their father (my ex) so I have no control over how he disciplines them (or doesn't, as the case may be) when I'm not there. I guess ultimately you don't have any control in that situation. Be thankful that your boy is good when he's with you. Obviously he has learned how to be well behaved when he is with you so it's not as if he is getting no discipline whatsoever.
Also, to play devil's advocate here for a moment, these people that told you your ex was letting him "run around like a monster".... are they friends of yours (or family) who have something against the ex? Or, perhaps these people saw the two of them outside and she was letting him blow off some steam (two year olds tend to have a LOT of steam
)....there could be reasons behind what you are getting from heresay. I would especially wonder if he behaves well with you. Obviously if she wasn't doing a proper job of raising your boy you would see signs of it when he's with you.