Marco is clearly incapable of realizing things that are stunningly obvious, but perhaps things that are slightly less obvious - like, say, the fact that he doesn't just love the Rat as his bestest friend ever, but is actually in love with him - he is capable of realizing? I don't even know how that would work. But Marco comes across as pretty smart, at least about everything else, so presumably he has some realizing capability.

Another theory: Marco's father has been brainwashing him from birth so he wouldn't realize that he was a Lost Prince of Samavia too early.

Also I love that you have basically made The Lost Prince your fandom of one. Tiny fandoms for the win!