@joannap-you are most welcome.
I try to provide what I have learned from ENTs themselves or from experiences shared. While I know you sought out an ENS expert, the term ENS is really outdated and only associated with surgery. However any change or alterations to the nasal cavity via surgical or medically can induce these symptoms.

Feel confident that that your son has options to seek out for restoration due to not being affiliated with surgery. Stay focused on proper total body hydration and anything that can bring blood flow to the nose, like cardio exercise. Highly recommend a functional medicine doctor who has an understanding of the nose as a full organ system and its function systemically with the nervous system. The nasal cavity functions best when it’s moist and many of the topical steroids dry out the nose. Steroids act on the cells in the mucosa by constricting their production of mucous. That’s why it’s important to focus on a cellular level from within the body.

The ENT who coined the term “ENS” actually worked at Mayo Clinic for 30 years. He was referred patients from all over. All complaining of the same symptoms post sinus/nasal surgery.
They all shared the same commonalities on their CT scans. A wide open nose with too much tissue/cartilage removed. Destroying the entire function of the nasal cavity. ENTs tried to ignore, and deny this. Or maintain that if they only performed “conservative” surgeries then this would not happen to their patients.
This still proved to be untrue as even to this day ENTs still don’t know how the nose functions and are just now starting to get a better grasp on the associated with immunology. It’s much more complex than what they have knowledge of.
This is not rare it is just underreported, because if a patient returns with complaints they are referred to a Neuro or elsewhere. So ENTs never report when they cause this and will continue to maintain its “rare”.

Anyone can read the Wikipedia link below.
https://en.m.wikipedia.org/wiki/Empty_nose_syndrome
And specifically the History tab. The only reason why ENTs are being forced to start acknowledging this condition is due to the availability of CT scans after 1980, internet exposure and more ENTs speaking out at their ARS conferences.

The nose functions systemically and is driven by the nervous system. It is meant to congest and decongest. That is part of its organ function.

I always have to maintain that 10% of patients who see an ENT will actually ever need or benefit from a surgery.