If his school's uniforms are anything like the marching band uniforms around here, I'd say yes they are very different. The poor girls in my town get saddled with these horrendous 1930's-style dresses, while the boys wear the traditional pants, shirt and braid-covered jacket.
If I were you, Mr. Fox, I'd just go to the teacher and tell him you'd rather wear the boy's uniform. No explanation may even be needed. ('Course, I'm the sort of person who would have said, "There's no way in hell you're getting me into that hideous thing. It's the boy's uniform, or I quit the band.)