It's not super posh. I can see him getting 'Maggie' at school and hating it, or being teased be ause of it, particularly (and I can't say this without sounding like a snob) in rougher areas. I know a Magnus here, who has never had problems, but goes to a very upper-middle prep school. Maybe its OK. If you can honestly look at the kids in your area and think he wouldn't stick out like a sore thumb, use it. Otherwise pick something else.
Not sure what 'lar-di-dar' means but I think Magnus is a great name.
Don't kids accept most names as long as they're not Poopoo or Willy? I mean, why would a kid at a London Comp care about chavness of poshness of names? I don't think so and I think some or us adults care too much about it. Lets judge people on their character, not their name!