Interesting question.  When I practiced on Long Island I referred to an
orthodontist who was also a musician.  (It was a long time ago--don't
even remember his name).
    There may well be other options, and considering whether your son is a
serious music student it may be something to consider--even if it
compromises treatment in some way (for example, it may take longer to
complete treatment).
    Another consideration is that the treatment itself will no doubt change
your son's embouchure.  But then again I've heard of horn players who
play with dentures, so I doubt the problem is insurmountable.
    We don't see orthodontists on the newsgroup too often anymore (sadly).
 But there are orthodontists out there that frequently face this sort
of issue, and I'm sure a good one will find a way to accommodate your
son's needs.

Good luck,
Steve

OR....Switch to percussion for a few years.  Marching band is even mor
fun when playing drums.

...How about the orchestra

Pick up the trombone again, after the braces are off

--
drdolittl