How can I convince my son not to let his truck idle in the driveway?
Maybe start by asking why he’s doing it 
Is it cold outside, and he’s warming the truck up?
Is he trying to do his part to speed up global warming?
If there’s not a great reason for him letting his truck idle, then maybe sit down at the good old computer with him and google “global warming melting ice caps”… tons of reasons not to waste fuel.
Think globally, ACT LOCALLY!
How long is it typically idling in the driveway?
What is the ambient temperature when he allows the truck to idle in the driveway?
When we were first married, we lived in an apartment. There was milk delivery in those days, and the driver let his out-of-tune truck idle below our bedroom window every morning. I could not get him to stop until I told him the next time I would turn the key off myself and drop it down the storm drain. That seemed to get his attention. Less drastic action as outlined by others might be more suitable for your son.
Is it causing a problem? Leaking? Causing irritating noise? Is he paying for the gas?
If the answers are no, no, no, and yes, then why not focus on more important disputes?
Remember, 99% of the things we obsess about turn out to be unimportant, and many of them temporary.
Oh, and of it really is causing a problem, go pull the keys the next time he does it. It is, after all, your driveway!
If he’s paying for the gas, there’s no reason to interfere.
The gas he’s wasting is less than the amount he uses to drive around on a Friday night with his friends, or less than the amount you use when you go back to the grocery store for the loaf of bread you forgot to get. Do you worry about that gas usage?
Is that idling causing extra wear on the engine or contaminating the oil a bit? Sure. But with the long life that engines get today, you will never notice a problem due to the idling.
This is one of those cases where you have to ask yourself: “Can you live with the consequences of what he’s doing? And if so, just let it go.”
sometimes up to 20 minutes, in all kinds of weather, from sub zero in winter to hot in summer
No he’s not warming it up, he lives elsewhere so it is when he comes to visit which can be anywhere from 5-20 minutes. Just long enough to use the computer and steal food from the kitchen…
It is usually his company van so it is not his gas. The global warming thing bothers me as do the fumes that might be coming in the house.
If you’re smelling fumes than go pull the keys.
Don’t worry about global warming. His contribution will be a teeny fraction of what Al Gore contributed when he took his cavalcade of 10,000 pound modified SUVs to his private jet and flew to LA to be carted by a cavalcade of monstrous limos to accept his Academy Award for his film on global warming. Whereinafter he traveled via the same methods back to his enormous mansion.
The 20 minutes in the winter makes a bit of sense, but it is still excessive. If he is doing it to warm up the engine before he drives it, he would get better results in sub-zero conditions by idling the engine for perhaps a couple of minutes, and then driving slowly. In the summer, what he is doing makes no sense.
It sounds like your son is lazy and wasteful, two traits that are not desirable, but will hopefully change as he matures. For now, you probably just have to put up with it unless you want to put him through the ultimate maturation experience, namely telling him that it is time for him to live on his own.
EDIT:
I just noticed your earlier clarification, “No he’s not warming it up, he lives elsewhere so it is when he comes to visit which can be anywhere from 5-20 minutes. Just long enough to use the computer and steal food from the kitchen”, and in light of that, I want to revise the answer that I provided above.
Even though he apparently lives in his own residence, he is not yet independent and he has not yet acquired the maturity that he needs to become a man in a real sense. Clearly, he is lazy, and yes, he is wasteful. But, in light of his apparent slothfulness, the only likely solution is for you to go out to the driveway, turn off the truck’s engine yourself, and hold onto the keys until he is finished taking food from your refrigerator.
You might ask him how he will explain the loss of the truck to his employer when someone steals it from the driveway while he’s eating/using the computer.
Scrabbler
Generally, I feel it would be inconsiderate to let the thing idle if it’s in your dooryard…like smoking in your house, not taking your shoes off. Whether it’s right or wrong, in my opinion, he should abide by your wishes. Whether you push it after he refuses depends upon how much you value his “inconsiderate” visits…
“Slothfulness” there were times when the church dealt out severe penalties for this “sin”,anything but slothfulness.