The Larsen family were a long line of Norwegian pure-bloods, one of the typical anti-muggle pro-purity lines who looked down on those lacking in magic. They had a son, Kristoffer, who happened to be a squib. So, at the age of ten, when they'd lost all hope of him showing magical ability, they sent him off to an orphanage and told everyone outside the family he'd caught dragon pox and perished.
In reality, the boy thrived, adopted by a well-to-do couple in England at age twelve - one of whom was a squib herself - and receiving high-calibre education in Eton and Oxford. Eventually he met the very pretty, very magically adept Viola Macmillan, a recent Hufflepuff alumna.
They married the next year, a few months before Viola inherited ownership of her parents' popular sweetshop, and eventually went on to have three children. The eldest, Magnus, spent a lot of his time alone, watching cheesy romance films or kitchy and camp comedies, thus resulting in a corny, cheesy sense of humour.
Magnus' first sign of magic was when he was about six, and he was playing with his brother, Kell, who would have been five, when he accidentally snapped Kell's toy. Kell started crying, and Magnus panicked. Before he knew what had happened, the toy had been fixed just like that, and Kell was happily going about his business playing with it.
Magnus' parents are slightly obsessed with him and his siblings living healthily, so while they're allowed as many sweets as they want, they have to make sure they do enough exercise that it doesn't affect them "negatively". Because of this stance of his parents', Magnus grew up playing Quidditch, taking part in athletics competitions and swimming clubs, not that he enjoyed any of it, and he tried to get out of it at any given opportunity.
At first, he attended Durmstrang with his siblings, as his father wanted him to "get in touch with his heritage" and have more of an opportunity to practise his Norwegian, which was limited. All was great there, and his Norwegian did in fact improve.
He's now ready and excited to finish his studies at Hogwarts.