First we tell him about each party's plans and goals, in age appropriate language, as unbiased as we can manage, as free of adult partisanship as we can manage.
We talk about our local candidates history and the party leaders. Using fair language, leave out the childish taunts from the Internet.
We explain who we're choosing and why.
Then of course, we took him to the polls. We told him who we'd decided to vote for and why, and we bring him behind the shade with us to watch us mark the ballot.
He's very interested in the process of voting and can't wait to vote for real. Hopefully he still is when he finally gets the chance!