This is a tricky one, I'll condense it down as much as possible.
Ex H and I have been divorced for 6.5 years, 2 children 9 & 7. We have a fine relationship, there's never been any physical abuse although ex h was/is very emotionally manipulative, is a liar (about stupid pointless things, lies to make his life easier, not sure why he does it, I've known him 20 years he's always been the same). We share custody 50/50 (well, nights 50/50, for example I have them every day during day in the school holidays, he just does the nights 50/50)
Our children, especially the eldest who is 9 has started to pick up on his dads lying and unreliability (note he's not unreliable about seeing the boys when it's his nights, it's more like saying he'll do things and doesn't, doesn't go to parents evenings, does go to school events, will randomly go away on holiday for a week or more without much notice, forgets things like reading/spellings/homework). From what I can see they both have a good time when they're with him, he does a lot of "fun" stuff, but less of the actual parenting I suppose.
On Friday after school after 2 nights at their dad's eldest son started crying and said his dad hit him on the leg while they were playing a game on the Nintendo and his dad "got angry". So from what I can gather it was a sideways hit out of frustration like you would if you were a child getting annoyed hitting out. He was upset and was saying "do you believe me? It's true he did hit me".
I asked his younger brother what happened and he said "yes that happened." Although when I later asked him again and specifically asked what happened his account of it was entirely different to what his brother said and what he has said the first time.
I obviously immediately contacted their dad who was quite shocked and upset by this, he said it categorically did not happen, they weren't even sitting next to each other when playing the game and seemed quite upset about what our son had said.
There's a few things to note here, ex h lies a lot, 9 year old son I've noticed the last year or two is picking up on this and emulating it, he will lie to me about things (nothing huge usually, similar to his dad) or try and convince me I've said or done something I categorically have not. Ex h has never been violent to me or anyone or anything that I've ever witnessed in the 20 years I've known him, he has a lot of negative behaviours but anger/violence is not one I've ever witnessed. Younger son can make up very elaborate stories about things and then after 10 minutes normally will say "that didn't happen I actually lied about that", they are also thick as thieves as brothers and will always back each other up. There's no marks on son and when I to look and check he said "no... can we do it later" "well it wasn't hard so there's no marks" (after saying ex hit him "really hard")I said I need to look it's important and he said "no can we do it later?" And got upset about me wanting to look. After looking there's no marks at all.
I don't know what to do or what to believe, I know in theory I should believe son, and that's the stance I've taken currently with him and ex h, but for us as a family and what I know of my ex this is SO left field and would be so so out of character for what I know of him. I don't even know what next steps I need to do. I really need some advice please.