John
The Explorer will certainly show the four main moon's of Jupiter, and the Great Red Spot under good seeing conditions (dark sky, steady atmosphere). Saturn's rings will be a great sight as they open over the next couple of years, and you will be able to see four or even five of Saturn's moons as well. Don't expect too much of Mars, it is a notoriously difficult object to observe, because it is so small. You'll certainly be able to see it, but surface features may be difficult to distinguish.
It will be excellent on the Moon (kids love the Moon, so start with that), it will split many of the best double stars, and will show other objects like the Orion Nebula, star clusters, and the Andromeda Galaxy. It will be ideal for sweeping along the Milky Way.
If he shows interest, and wants to use the scope regularly, get him to keep a notebook or scrapbook of what he has seen. Include cut-out pictures from astronomy magazines and also his own drawings.
Also, get a copy of this month's "Astronomy Now" magazine, there is a 12 page section about getting started in astronomy. Ignore the suggested telescopes to buy, I think they are aiming a bit high, but there is a good section on things to observe.
