When I worked in a children's home, our response was to interrupt the child and ask:
Is someone being harmed?
Is something being damaged?
Is the building on fire?
Then what you may be about to say could be tattling. If it is, you will receive a penalty for tattling. Now, do you still have something to say?
The kids very quickly learned what sorts of things needed to be reported.
That's basically what i have used with my kids. I drilled it into them from an early age that tattling was unacceptable and we had to ask ourselves :
"Are you trying to get someone in trouble or out of trouble?" In other words if someone was about to get hurt, or hurt someone else ( Lord knows I don't want to be that parent who thinks it's always someone else's child who is the problem ) or otherwise have a really bad outcome then you can tell. If you're trying to get someone in trouble then I will not listen to it.
My kids are ten and six. Trust me they still try to tattle on each other.....but it gets shut down quickly when I ask "Are you ratting?"
I would do the same for this kid - maybe not in those words but do not reward a tattletale with anything that gives them any sense of importance. No need to be harsh with a child - but you can definitely tell him that you don't listen to tattling in a neutral voice....and when he hasn't gotten his satisfaction you can decide whether you need to intervene with your own child.