@Invader_Retro I unfortunately didn't. The king knows that the MC is doing something that he stands to gain apparently nothing from. So right off the bat it's obvious that the MC has other plans. He should also be aware that the MC knows that he'll try to kill him to silence him by that point (since once again, he becomes an inconvenient person to keep alive - in fact he's not convenient to keep alive even before that). This obviously leads to the conclusion that the MC believes he'll get away with it. The king is supposed to have figured that out, and has no reason to go along with his plan - he could have just as easily told all of his children that the MC's playing them, and then the MC's done. The king has no reason to play into his hand - his children are a lot less dangerous than the MC and the king knows that.
@Kayriel I am not assuming everyone else is hyper-intelligent, I am working off the premise that they are not retarded, which simply doesn't check out. There's nothing hyper-intelligent in what the MC does - he'd have failed a thousand times if everybody wasn't just magically playing into his hands for no apparent reason. You're mistaking education for wisdom also. And the king should be a lot more well-versed in backstabbing, deception and power struggles than our MC. So it's pretty unrealistic when he just agrees to the MC's stupid plan.
You're comparing this to the human king to boot. But that's circular logic - the human king being dumbed down, in order for our MC to succeed, doesn't make it any more acceptable for the lizard king to be dumbed down. If you look up the definition of Gary Stu, you'd notice an uncanny similarity to our MC, and to how poorly the story has to bend for him to succeed in everything he does (forget plot armor, this is some next level plot device magic we have here).
@yarielist It's precisely the fact that power is everything and that you can get dethroned at any time that vouches for the fact that the king deceiving everyone is not king just by chance. He should be used to backstabbers by now, and know quite well the motivations of those around him. Instead he's dumbed down to the point where he believes that the MC has his best interest in mind, and plays right into his hand. He already knows what the MC is capable of and he's no stranger to deception and magic, so him playing along shouldn't be happening. But The Gary Stueness of our MC requires that the story bends over to make him look like the master puppeteer that he's not.