One time a 7 year old snitched on me. One day I pulled him aside and had a man to man with him. I told him we couldn't be friends if he didn't understand what's said between the two of us stays between the two of us. I told him I lie a lot to protect people's feelings, although I speak candidly around you, that doesn't mean you can tell everyone or call me out in public. Secrets are to remain secrets.
This is the dad's fault for not raising his son better. Kids can understand a lot more than we think. Communicating ideas to them is tricky but it can be done.