I don't like the flow of the story here.
He was just thrusted an opportunity to do exactly what he has been trained for most of his life to do and with possibly the best standard of living, so the MC just jumped at it. Any perceived potential danger was just sorta written off as an occupational hazard.
Is this a good idea? who knows. Did the MC ponder other possible options? Nope.
As for the kingdom... Regardless of his background the kingdom should have stated out by having him guard lesser nobles, with others to surveil him, to better judge his character, motivation, ability, and trustworthiness. And only then appointing him to royalty. This is way to careless. Either the MC is set up/betrayed or the author is just stringing along conveniences.