"Vehicle" based MMOs like World of Tanks or the already mentioned Elite Dangerous, but not everyone will be interested in these games.However, regarding other kinds of games there are so many titles to choose from.
There are several ways to earn Linden dollars in big quantities, and later sell or exchange them for real-life dollars.
One way is to create mesh - basically, design or sculpt products to sell in the marketplace for profit.
with your avatar you can walk, run, drive, teleport, meet other players, buy land, build property, create, design, sculpt, have sex, even marry.
That's the most basic summary of how Second Life works - but it can also get a whole lot more complicated.
Maybe go for different genre until he grows up a bit?