I've been working the same situation with my son for the last couple years. He is also 8. I think the best way to train is exactly what your doing, a place with a map would almost be ridiculous to use b/c its so well laid out.

I have worked with him on the most basic type trail maps, the larger the better for a small area. For city travel he has a map with the streets on it so he can follow along. Also, 21st Century cheating, he uses my iPhone map system to follow the blue dot to the red dot as we drive. (would need to know the iPhone map to understand that, perhaps).

He really got the understanding of a map, (finally) when we were at Disneyland last year. As all the rides are laid out on it. We would pick a ride, then he would orient the map based on where we were and he'd follow the rides on the map with what he say in front of him. Worked really well.

Maybe a county or state fair near you, (or a theme park) would be a good really fun diversion to get him to understand ready a map and orienting himself to it.