Being accused of allowing abuse in my house

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This is my third username, I've lost my other accounts.  I was Frankee and 2Strong2Long.  Update on my situation.   4 and a half years out of my relationship. 

My ex now has unsupervised visits with our son, once a month, 4 hours.  He also has weekly phone calls.  Last night he did his phone call (disaster).  Our son told his Dad that his big brother hits and kicks him when gets mad.  Not like punting a football type kick, but enough to where it hurts his younger brother's feelings.  This is something that big brother has gotten in serious trouble for and the issue has been addressed.  We've also had family discussions in regards to this.  The problem has gotten better, but big brother still has anger issues I am helping him to work through.  There is no excuse for big brother lashing out and I'm not excusing him in any way.

However, my ex has now taken that tidbit of information and now thinks big brother is running around, abusing and beating up his little brother on a regular basis and I'm aware of it and letting it happen.  It doesn't help that my ex was abused (certain kind) by a teenager around big brother's age.  So of course he has transposed his trauma onto what is happening between the boys.  He told me that if our son shows up for visits with bruises, he's going to be pissed and that he's not going to "put up" with it.  Then he also proceeded to tell me he knows I've told out son to lie to him (which I haven't).  Well, he's a boy.  He gets bruises all the time from rough play, either at home, school, or daycare.

I've already talked myself through my feelings and I'm over it.  I also had to remind myself that my ex in a looney toon and isn't going to let this go easy.  He's been fishing for information to find out anything I'm doing wrong to use against me.  We've been down this road where he tries to make everything harder on me because I feel he's just big mad that everything went down the way it did and now I refuse to argue with him about anything.  I googled the reasoning an ex picks fights and what I found describes him to a point.

The purpose of this post mainly is to get feedback on other's either going through or gone through this type of situation.  I know I do not need to admit any type of guilt (because he will run with that), but this is actually new territory.  I've never had abuse accusations thrown my way.  I am also now concerned of the next time I have to face him in person if he will attempt to confront me about what our son said.