QUOTE(Bobilleg74 @ Jul 18 2010, 06:41 PM)

The Bach Prelude is basically a starter instrument. If your son is Grade 5ish and looking to continue to progress further, I would suggest that now is a good time to think about upgrading.
The thing with the basic student models is that intonation isn't great and the tone quality won't be either. As to how it plays, I would argue that it will be partly a technique thing on his behalf, particularly as you mention range and intervallic transitions. I bet if you put the horn in the hands of a pro, they wouldn't have too many problems making it do what they want!
I'd say that unless you've got a really duff instrument (unlikely - most of the modern ones are OK if they are mechanically sound), the sound that comes out of a trumpet is 80% technique and 20% instrument (though agreeing that intonation does vary between models). I use £100 Packers student trumpets in lessons partly to demonstrate what is possible with a cheap trumpet. I don't get the 'Grade 5' thing, as grade 8 would be perfectly possible on a £100 trumpet if the technique is up to scratch.
Having said that, £100 won't get you an
exciting trumpet, but getting a £1000 trumpet won't necessarily make high notes or lip slurs any easier. It might be worth trying other mouthpieces first, although there's nothing inherently wrong with the standard Bach 7C-type of mouthpiece.