A suggestion. You might describe the scenario to the child. If he has a friend who suddenly changes in behaviour. Then what would he do?

The harder part would be explaining that while you have issues with the mother, he is not to assume that you are judging the child or mother.

The hardest part would be telling him what to tell his best friend why he can't come over. If you don't like fibbing, your child will hate lying to his best friend.

If there are only five weeks, you might need to fake an illness so that all parties are given a face saving explanation.