Blond haired boy goes in room to find a huge present and inside is a red convertible car, when he gets into of car he flies out of window and the movie turns into a cartoon and his adventure begins... does anyone remember this!? At somepoint he meets a "weather man" and the weather man floats off holds a bunch of ballons and says "I hope it doesn't mildew"

Milo had brown hair
But it sounds like,.

The Phantom Tollbooth (http://www.alligatorjuice.com/cgold.html)