Mind-control the king into packing the kid away to a monastery (thus disqualifying him from the succession) and take him from there. Once the kid is off in a monastery, being Gifted, the king probably doesn't want much to do with him anyway, so this should not be a hard sell.
Aside from the obvious consequences, I would note that a sufficiently-strong magus can take a throne fairly easily; obliterating armies with a single spell is well within Hermetic power. The problem is, if the magus' gift isn't Gentle, holding it will be near-impossible. A king lives or dies by the personal loyalty of his nobles, and a Gifted king can't get that easily if at all; even the peasants will be whispering about the Satanic powers he used to take the throne, calling him a changeling, et cetera. Between constant conspiracies by his nobles and general unrest throughout the land (and you can bet that demons will be fanning those flames with every random bad harvest), the realm is likely to descend into anarchy; if the magus has to crush the lords who rebel against him, then he has nobody to run the kingdom.
It also bears noting that every season a magus spends running a kingdom is a season that he isn't researching, and that a magus-king both has responsibilities to his realm as king and to the Order as a magus. If he's lucky, being a king will provide enough benefits for his covenant that they'll accept mundane lands and income as an acceptable scutage for his covenant duty under ordinary circumstances. So in the best-case scenario, he's a Wealthy king who can spend a season a year being kingly and spends the rest of his time lab-ratting like everyone else (hell, he might even have a nice Magical aura to research in)...but being a king is almost as good as being in Faerie when it comes to attracting adventures. Most years, he'll have to deal with a summer campaign season as well as a winter court - there's a reason that covenants leave this crap to the autocrat, but an absentee king is absolutely asking for trouble. And then, being a magus, he has to deal with Hermetic politicking, which means adventures spent pacifying Quaesitores, providing royal support to his covenant, dealing with Tytali who get the bright idea that they can take his throne now that he's set the precedent, and did I mention pacifying Quaesitores?
Also, it does bear note that once this genie is out of the bottle, certain species of Jerbiton (and those of similar inclinations) are going to get ideas. Thebans might want to restore the Empire, Levantines might accelerate the Crusades to carve out their own kingdoms, the Holy Roman Empire is approaching anarchy in this time period (especially when the Emperor and Pope start going at it), Iberia is a free-for-all...if one magus can take a throne and make his claim stick, it'll set a precedent that'll lead to magi attempting to seize thrones all over Mythic Europe. How will the Church react? How will the Dominion react, for that matter? The kings that the magi toppled were appointed to rule by divine right, and God can hardly ignore it if magi are suddenly unleashing their power to upend the entire social order. And...how will magi react to each other when their domains start coming into conflict? IMS, part of the reasoning behind the rule against mundane interference (and part of the reason why House Tytalus wants this part of the Code burned to the ground) is because Trianoma wanted to prevent turf wars between empire-building magi.