E-5 Promotions have mostly been taken away from the units when a soldier hits their primary zone for promotion without waivers.
If you don't provide a solid justification as to why a soldier is not NCO material, the Army will place them on a centralized promotions list.
For E-6 you still must appear before a unit board. There is talk of centralized promotions to E-6. The Army is not happy with leaders not promoting their soldiers so that is why it has been taken out of their hands. It is necessary for the Army to do this to maintain NCO's and junior level leadership. It is a pain in the ass due to the fact that soldiers that are promoted on time in grade and service, not leadership ability and potential.
I have a soldier that came to me from another unit that was on "The List" and he's fairly incompetent as an NCO. My NCO's that work for me earned their stripes by appearing before a board of 1 CSM, 3 SGM's and a 1SG, all of which are Special Forces Soldiers.
Unit allocations are MTOE specific. For strength management, A unit is allocated a certain number of soldiers by MOS, and Pay Grade. For some soldiers to be promoted and maintain a healthy career, they must demonstrate potential at the next higher pay grade and have an appropriated slot for them to work in. This keeps SSG's from being rated as a Soldier filling a PV2 position (a career killer for support troopers).
That CSM is upset for manning reasons, not at your son. He may hate Cav Soldiers, but it's your son, he comes from good genes. I'm sure he'll do an exemplary job.