I assume that he has been evaluated for esophageal strictures. If so, strictures can be treated with repeated dilations. Unfortunately, post radiation induced fibrosis is an ongoing process. Increased fibrotic tissue may oft times form adhesion and entrapment of the neck and throat musculoskeletal elements and their innervation that may impair the oral, pharyngeal and esophageal phases of deglutition (swallowing).
Swallowing exercises may be helpful depending upon the severity of fibrosis.
I've had MS related achalasia for > 30 yrs which has been further complicated by my diagnosis and aggressive treatment of stage 4 squamous cell esophageal carcinoma.
I developed sever post radiation fibrosis and stricture formations that rendered me non-PO for liquids and solids. I had G-tube placed 3 yrs ago and it markedly improved my nutritional and hydration status. I was able to regain weight from 122 to 162 lbs over several months and have maintained that weight ever sense.

I was able to find a thoracic surgeon who was able to successfully dilate my esophageal strictures to allow to me to eat and drink again. I have repeat dilations every 3 months.
I still have my G-tube in place to use when I become progressively dysphagic between dilations.

I am also fortunate that one of my daughters is a speech language pathologist who specializes in swallowing disorders associated with strokes, TBIs and cancer. She has assisted/ instructed me with various effective swallowing exercises.