Tell him its a very powerful natural force. It is energy found in everyone and everything. It works to influence, not make instant, dramatic, physical changes. It is a force of nature and therefore works within nature.
If he asks you to prove it that's difficult as it comes down to belief and its not something you can see, only feel, but you could give him a charm for luck, even if he doesn't equate his good fortune to the charm, what's the harm?