QUOTE(frumpybabes @ Apr 20 2008, 11:40 PM)

My son (12) was playing on a 7C Bach but since acquiring a xeno trumpet, he's started to use the xeno mouthpiece. He got gr7 last year with distinctionn and is now preparing for gr8 so I would say he is starting the advance stages. He plays in orchestras, concert bands and big bands and has been advised to look at getting two new mouthpiece for improving his high notes. He can hit D but would like to get higher and be able to sustain the sound for longer.
Actually, the Yamaha mouthpiece supplied with the Xeno is reasonably large (I think the number of it is 16C4) so will not help the high notes, although this would be OK for an experienced player with a well developed embouchure - but at age 12, this is a big mouthpiece. There's not much wrong with a Bach 7C, though the rim can feel a bit sharp to some people (me included) - I prefer a Schilke 4 rim. With my Schilke 17D4 (this is slightly bigger and deeper than your son's Yamaha) I do classical work up to concert E (written F#), but then I've had 30+ more years to develop my range!
So yes, a smaller mouthpiece might help - I use a smaller one for lead trumpet in big band, which often sails up to written G on the 4th leger line; my Schilke doesn't work in this context, as it is both too hard work, and doesn't cut through a band. John Miller's advice (apparently) is that one should use the smallest mouthpiece with which one can produce the desired sound - why waste extra energy playing on a big mouthpiece?!
If you want a chart comparing the size of mouthpieces, try this:
http://www.militarymusician.com/xcart/pages.php?pageid=1But, as I said, don't expect to find the 'miracle mouthpiece' - it doesn't exist. They are all trade-offs - usually what you gain in range, you lose in tone and flexibility.