Reddit reviews Be a Changemaker: How to Start Something That Matters

A couple books that might help him get started - For environmental issues, or just for a good template to follow more broadly, I love the story Follow the Moon Home about a class taking action to protect sea turtles. You can see a good outline of their approach in one of the illustrations in the amazon preview, a couple pages in. For more societal issues, and aimed at slightly older kids, you might check out Chelsea Clinton's It's Your World: Get Informed, Get Inspired & Get Going!. Be a Changemaker is aimed at teenagers, but might be something you could read through with him.

Not sure I can suggest much more specifically without knowing what issues he has in mind, but it might help to join up with a local organization that supports his particular cause and attend some of their events, help with fundraising, etc. There are TONS of opportunities out there for kids to get involved in lots of different activism efforts.