My kid didn’t ask whether the two extra music letters (one from teacher, one from youth orchestra conductor) could be submitted, in addition to the two academic letters. Kid just sent them, college took them, and they most definitely played a role in the committee’s evaluation - kid has seen the file, in which the committee quoted from the music recs. I was told that the entire committee listened to kid’s 3 minute music supplement, and that they sent it along to the school’s conductor. But that school truly values performing arts on campus, even though they don’t have performing arts majors, and for this reason it seems that a lot of the successful applicants there have very high achievement in performing arts, in addition to academics.
So just send the two extra letters and the music supplement (3 minutes of solo playing is long enough to demonstrate his skill, and short enough that they’ll actually listen to it). Beware that it is extremely difficult to manage enough practice time to justify private lessons, while double majoring in music and academics. Once your son is at school, he will figure out the balance between what he wants and what he can manage, and the role that music will play in his college life.