I agree with everything PinkTulips has said.
I can't lie to ds1. I can't give him silly ludicrous answers the way I can with ds2.
I have a friend who, when ds1 asks me a question such as "What's for dinner?" will butt in with "fried dinosaur!" or something equally silly. Her children and my youngest will laugh their heads off - ds1 just looks lost, and reiterates - "Mummy, what's really for dinner?"
he knows that that's a joke - he doesn't get it, but he knows it's a joke and not meanness. But some of the subtler lies would really upset him when he found out the truth. He knows that lamb chops come from lambs, which are dead baby sheep. He chooses to eat them anyway. He knows that when he goes to his dad's, he is coming back at 5pm on Saturday, not 'soon'. If someone says to him "ohhhhh I'll take you home with me!" I have to quickly interject with "ds1, X is joking, you are coming home with me like every day".
He's not a child for subtleties, and rather than have him HUGELY upset when he realises he hasn't got 'the joke', I would rather tell him right here, right now, what is true and what is not.
Interestingly, he pooh poohed my insistence that guinea pigs were real until I took him to a petting farm and showed him one ! i guess it sounded like one of those ludicrous jokes people play on children ....