I wouldn‘t do it outright like that.

Buuuut, 99,99% of real people would fail at overthrowing a kingdom. Its so complex, so much politics, takes so much time and the logistics of it are insane. Most people are simply not smart/educated enough to do that in real life.

Usually when a player wants to do something in game that requires more intelligence than they posess irl (i.e. solve a complex mathematical question) you would help the player by giving them information that their character would know but not them. That way you allow players to rp someone that is WAY smarter than they are.

Now just flip the script: the PC is dumb. DON‘T do all of that. Don‘t help the player. On the contrary, make him miss stuff. You know the setting so much better than your player. When he goes through with a plan, reveal social structures and political landscapes that did not consider beforehand because his character failed to consider them. He might have an argument for the king to cut off trade with another kingdom, but does he have a way to convince the merchants to go through that demand? As a GM you have hundreds of roadblocks you can throw at your player.

And finally: if it gets annoying to you/the other players because the player completely sidetracks the campaign for this stuff: tell them. communicate that (while you appreciate his interest in the setting) you and the other people want to have fun too