Based on your grandson's background, he might qualify for one of the service's Premier Bands. In the case of those, I would recommend he first see if he can find examples of the skill level by listening to YouTube videos of these bands.
The Premier Bands are The Air Force Band, The Army Band, The Army Field Band, The Marine Band, The Navy Band and The Coast Guard Band. Please note that these are ~THE~ ___Band and not ~a~ ___ band. Also be aware that the Marine Corps is part of the Dept. of the Navy.
Additionally, each service has its Academy Bands which are very good.
For requirements and more info, he can go to the web sites of each band.
You can also get similar information on the Fleet Bands and Field Bands by going to the respective service's web site. For example.
https://www.music.af.mil/
https://www.bands.army.mil/
etc. He can google the others.
(in this context "field band should not be confused with the Army Field Band by the same name.)
If he lives in proximity to any of these bands, I encourage a trip to it (them), listen to the band and talk to the band's band recruiter, who is a bandsman NOT a local generic recruiter.
It is important that you do not initially talk with any of these Recruiting Service Recruiter. That will come later but that recruiter may not be aware of programs. policies and other programs specific to band programs. . They are generalists and may not be aware of the finer points regarding military musicians. Deal with the local band recruiter and the headquarters band office in D.C. (D.C. in this case is a generic term to signify that it is the Headquarters. Physically. some bands or music-program offices may actually be located elsewhere.)
Not to mean that the Premier Bands are not superior, but the field/fleet band programs of today are not your grandfather's bands.
Over the years, band programs have been progressively reduced. What that has meant is that. as bandsmen were numerically reduced, whatever deadwood was there has gradually been eliminated. Nowadays it's not unusual to find a high number of college-degreed bandsmen in the non-special bands, some even with Doctorates.