If it doesn't cause a rift with your wife, have your son come with you when volunteering.
Also, make sure to carefully plant little seed in their minds, without ever mentioning the JW or WT. Use the examples of say, government and how wrong you think that certain thing is, or the media, or another religion, but it is an action that the JW or Watchtower, or GB uses. It will cause the cognitive dissonance in their minds to be strong, sometimes they will initially agree with your viewpoint, then stop talking. They recognize something, but not sure why sometimes, or sometimes if you are lucky they will actually recognize the JW equivalent action in the example.Once you see, (and you can actually see in the face,) the discomfort, it is time to stop and bring up some other subject . Otherwise it is too much.
You are in the position of exposing your son to the world outside the JW so he can form opinions by actually seeing or experiencing things, such as how non-JWs can be truly kind and happy.
Disclaimer- From a never has been, never will be JW, but married into a family of them with a fence sitting unbaptized husband.