I think he's probably just testing how far he can go in playing his mum and you and his dad off against each other. My step kids did this for a while and I just ignored it and it went away. They would come to me and say things like "my mum says that the washing powder you use give me a rash." Sometimes it was probably something the mother (who is very evil) did say and sometimes it was just something that they had made up. I would just answer something like "oh, that's too bad, why don't you get her to send down some of the one that she uses and I'll use that instead." I would never give it credence by checking with her whether or not she had said it as either she had and it was a dumb thing to say and my telling her that would just get her even more riled up, or she hadn't said it and then she'd get riled up that I would have thought that she had.
I think that all kids (even the good ones) do a little lying at some stage. It's part of the pushing boundaries that happens as they grow up. Don't encourage it, but don't blow it out of proportion either unless it's about something serious.
Also, I think that your son may also be trying to ingratiate himself with his mother by telling her mean things about you and your dh. My stepkids do a lot of this to their mother because she encourages it. I think it's unhealthy and evil, but no amount of my telling her so will ever change her behavior, so I don't say anything just do the right thing when they are in my home.