Playable for a 5 1/2 year old boy?

[Stegei]

The game is primarily a sandbox game, unless you do the separate tutorials or the optional helper missions in the career game.
There is not a lot of text to read. So once you understand the controls and know what each vehicle is supposed to be doing, you're son should easily be able to play it by his own with some experimentation.
The full game mechanics in the game are more complex than in Euro Truck Simulator, but I guess he will already get like 80% of the fun by just driving around with the tractors which is not more complex than for Euro Truck Simulator. And as he's learning more, he'll be able to gradually step further into the more complex things.

[Hairy Bob]

My 6 year old gets on fine with it, once I explained the menus and what does what. I've also got a 2 1/2 year old who loves it, playing on one of my old saves that I left with a few million in the bank. He usually buys a new tractor and drives it around flattening all the crops!
It would probably be worth familiarising yourself with the game, so you can answer any questions that come up.

[Bradleyson]

Indeed, my 6 year old daughter loves helping me cultivate fields who needs the ai workers?