Hi,
This is not a fun situation, but from my point of view, there is nothing a parent can really do. As an example, my own mom taunted me one time, by saying that she would pay for my education at UCSB if I studied Portuguese, knowing darn well that I was into Theater and Film, but she didn't care, she wanted a carbon copy of my dad to "continue" the structure of the "star" in the family that made it all possible for any of us.
I can understand the parental this and that, but in the end, forcing the kid, is not going to help and all it will do is get the kid into a "forced" process to succeed and eventually, it all gets thrown away, specially if the road is not easy and it takes longer than originally thought, which you and I know, could take a day, or a lifetime!
I would sit down, and have a solid talk ... and explain that to learn an instrument, any of them, takes forever, but if he listens to radio, tv, or his iPod, ask him how many horns can he remember and mention in songs, for example ... and he might end up thinking that dad is on his side, but would not want to see him struggle and get nothing for it. The chances of us seeing another Miles, or Dizzy, are next to nil, because the music industry, specially in America, is about money, and those were known, and are known, to not make that much money, which means, you are not going to find gigs a whole lot, and maybe showing up at the local bar and hope that you can jam with jazz folks and make it work? ... good luck, since they are also fighting for their own spot, and giving it away to you, is not an option ... they are the band selected.
In the end, any career in the arts is not fun, and more often than not too much happens that is very difficult to deal with, and it gets harder when you turn 18 or 20, and all of a sudden you have a girlfriend and then a child, and you have no steady gig to help pay for it all, and life all of a sudden, has a tendency to kill the music, and only allow most of us to simply do it on the side, hoping that one day we can get our own 15 minutes, and that we're good enough to turn it into 30 minutes so we have a chance to stay there.
I don't like to say that it's all a crap shoot, or a lottery win, but it's best if we're honest about that, instead of hopeful and dreamy about it all. This was a very serious concern at UCSB, when the folks in the Advanced Acting program were all being told they would get a "tryout" at the end of the year with professionals, and when you went there, it was 3 sleaze bags, one reading a Penthouse magazine, and the other smoking a cigar, and the third sleeping on his chair ... and you are expecting those folks to "choose" you or anyone else? It was a scam ... plain and simple scam, and every year, some 3 to 4 thousand acting students pay for that ... and then they go back home ... don't call us, we'll call you ... and you have nothing to do other than some local cheap theater that doesn't pay, and only does the same crap as the local high school.
The last option, of course, is you get your ass down to ________ boulevard, and scrape and scrap a gig, and somewhere along the way, things might click ... but remind him, that there are hundreds of others trying the same thing, and they are in the same shoes ... simply trying to find a meal and a bed!
Good luck ... this is not meant to be a down thing, but it is meant to be a wake up call ... even the "best" don't always make it, regardless of the instrument they play, and the reality is ... you have to know what you are doing and be better than the others, and that is something that no father can teach his son or daughter ... they have to develop that themselves.
Make sure your kid knows that ... and the sooner he/she understands that, the better! But you still have to love him/her ... regardless of choice. It's your child, after all!