Update: Mattias said that it's not him. He also said that they probably think it's him and that's why his name stands here: Youtube (0.35) But come on! They should know who sing the song, so I don't think they just guess it's him!
Okay so I got a mail from Nanna and i know a lot of other people think the same ;)
The thing is, that the themesong in Phineas and Ferb sounds like it's sung by Mattias!
Well i don't know. Yes i can hear that it sounds like him, but still i think the voice (from the themesong) is a little darker... But then again it really sounds like him :D!
But what do you think? Is this Mattias?
Maybe we should ask him...