What a night mare! I hope he is telling porkies in a weird way!
My DS was 2 in August, 2 weeks ago whilst my DH was away for 6 weeks for work he told my mother that "Daddys hits mummy, mummy cries, Daddy shouts, Daddy leave Daddy no come back"
Now, I was with them at the same time and even though none of it is true I felt so strange trying to tell my mother (who also knows it is not true) that he was joking! I had to go into nursery and ask if he had said anything to them, I was so worried and then felt it look like I was prostesting a bit too much!
This week he has told his keyworker Mummy is sleeping at Tim's house, Tim is one of my best friends and I did stay there a few weeks back after a big night out - Thankfully his keyworker found it hilarious that my 2 y/o is basically telling everyone my DH is beating me up and I'm sleeping around! If you knew me or DH you'd know this is totally laughable now if he was telling them I was beating up DH ...
Maybe he is just testing the boundaries or has seen something on TV about it? Maybe one way to talk it through, once you have made sure it isn't true, would be to speak with the Nanny and then both talk to him?
I also work with a young lad who is 22 but has a lot of learning difficulties and special needs, he once told his mother I had smacked him, she quite simply said "You probably deserved it" - She then asked me about it, I said, obviously, that I hadn't and wasn't sure where it had come from, she said they had been watching some soap where the mum had slapped the son and he had taken it quite badly so perhaps he had picked it up from there - Thankfully we laughed it off and 5 years on I'm still working with him