I played for many years... Mostly Jazz, I also played Contra Bass with the Blue Devils Drum and Bugel Corps back in the late 70's and early 80's
as they say "LOW BRASS KICKS ASS"
I dont play much anymore but still pull my valve trombone out and occasionally one of my two turn of the century 1900-1920 Silver Conns out.
Is he looking to play a bass trombone or a Tenor with an F attachment?
If he has a current tenor of a known quality brand you might be able to just purchase the F attachment.
But with a student trombone you should be able to get one for much less than $1800.00
If you are lookin at symphony/concert grade equipment, i would try to make that investment after he gets his musical direction this year, he will have many options to chosse in high school.
You could also query the local Symphony or Musicians union to see if there are any good horns for sale.. or Craigslist...
King and dynasty also put out some very nice equipment..