My daughter was told she could try to apply directly to PhD programs but she wants the extra time to develop. She had heard from mentors, when an undergrad, that she had a strong individual “voice”. She could have left it at that, but her thesis reflected those same mentors shaking that status quo up and challenging that “voice” by introducing electroacoustic elements and she grew by leaps and bounds. Graduate school should be about that kind of challenge and evolution.
She feels that at the PhD level you should really know your focus, as well, and no matter how competent a student might be, or how strong their vision, a master’s can really help narrow your purpose for the PhD work.
I guess the simplest way to put it is that if funded, a master’s just gives you that much more time to grow and develop as an artist. She made funding a priority in choosing where to go, of course, and that can be a huge factor in deciding to go directly to PhD, since more PhD programs are fully funded than master’s.
Your son is gifted in many ways and obviously has a lot of stamina too. He is also very proficient in theory and other technical aspects of composing. If he knows what he wants to do compositionally, there is no reason (in my view) not to go ahead with the PhD. If he wants to get to teaching or whatever career he has in mind, then it would seem he should go ahead with the PhD.
And then some really desirable schools only offer a PhD, so that is a factor too in going ahead with the PhD: access to certain schools.
I cannot really think of a strategic downside other than fewer years to make connections and hone craft. If I see that someone went directly to PhD I figure they are pretty darn good and/or they really have it together. But as I said, my own daughter doesn’t want that.
The only other thing, just rambling here, sorry, is that the choice of PhD program schools might be wider and better after a master’s. Doing a master’s might even change which schools he would want, if his aesthetic changed in the course of the master’s.
My daughter considered alternatives to a master’s to get her to a PhD program at a similar stage of development. Meaning, say, working as a composer, applying for grants, and getting pieces played as much as possible. or going to Europe to study with one of her favorite composer teachers. Trying to get residencies. Finding a studio to work in and learn more about electronics. Studying art history to enrich music. Going to Alaska or Japan for inspiration. Etc. etc. But in the end the very best thing is to have the structure and resources that a school provides.
Another issue is terminal master’s or not. She is choosing a school where master’s classes count in the PhD program. But there is a terminal master’s in case she does not want to continue on to a PhD level.
If your son likes to teach, and it seems he does, a final thing to think about might be teacher training and opportunities at the master’s level Funding for PhD programs involves teaching duties, and I don’t really know how many master’s programs do too, I just know that some do, some don’t.
There are some interesting connections these days in some programs, between neuroscience and music (see Dartmouth Digital Arts for instance). Does he have any interest in pursuing a dual focus or combining his interests in some way? Would master’s work help clarify? Has he done any electroacoustic work, or is he interested, and does he have experience? Is he pretty sure of the direction he wants his work to go?
He may very well be ready. My daughter likes to be a turtle versus hare and wants the time, but your son has already gotten a BM as well as BA and she did not have the full BM experience as an undergrad, just a good rigorous BA in music.
This response is not very well organized 
Good luck!
ps. gradcafe.com might have a thread on this, haven’t looked but it’s an idea; also newmusicbox.com
