Ouch! Firstly that's a real pickle of a situation! Secondly well done for not being the one to badmouth your ex in front of your children as this is obviously an immature way to behave , as for how to handle the situation is also a tricky one but you must remember children are easily led and influenced by others especially background parents ( mothers fathers that have access to there children but not residency ). Also sometimes a problem is these parents can tend to try overcompensate for the lack of access and basically can get more quality time than you with a child.. BUT always remember the TRUTH will always come out in the end!
Firstly I personally ( and this is just an opinion ) would be looking to rip a strip of the ex and let him know what a stupid and immature way to behave ( as come on it was ) , then you must decide to either sit down and try talk out some of the truths about his father with him again ( and obviously at your digression how much details of what REALLY happened or elaborate on what you have already told them).
A lot of things in my mind as to conversations with your son ( again its just an opinion) depends on WHAT you have already told them about the separation, and how much 1) they need to know 2) they can handle as an individual .....this would be up to you to evaluate and possibly discuss with any current partners that are /or may be involved in your sons life.
But definitely fight your corner and present the kids with the facts JUST THE FACTS, and they will either believe you or not I'm guessing ...But even if your son doesn't believe you right now again the truth will ALWAYS come out( usually as the children mature and start to question the past) and when it does that's when to calm and reassuring to the children.
Like I said these are just my inputs and opinions but happy to throw my thoughts out there