Joined just to comment on this as I had something very similar myself at the age of 12 - I was scared of choking on food, always felt like there was something stuck in my throat, felt like I would randomly forget how to swallow, and would sometimes have to go to the bathroom during meals to spit out food. I also felt like I couldn't explain what was happening so would just say I wasn't hungry, when I wanted to eat properly, I was just too anxious to do so. I would also have episodes of feeling like I couldn't breathe, I knew I was breathing but I was convinced I somehow wasn't getting the oxygen. This started after I accidentally swallowed a mint (didn't choke, was just uncomfortable but couldn't stop thinking about if I had choked e.t.c.)
Something that helped me was being distracted from food so I wasn't focussing too much on my own body or the fact that I was eating - I could eat a full portion of breakfast most days, because this was serve yourself at whatever time you got up, and we were allowed to eat in front of the TV. I could eat certain foods at school as well, distracted by friends. Eating in front of family at dinnertime was the worst, as it felt like pressure, the focus was on eating which made it feel impossible.
If he doesn't already, possibly try letting him eat while watching TV e.t.c or anything else that might distract him. I also found listening to music, radio or being involved in a lively conversation (but not about food!) helpful, it was the mental distraction, and being able to listen to something to drown out the sounds of my own chewing/swallowing.
If DS has siblings, the worst thing you can do is enforce 'no-one leaves the table until everyone is done' - this was the case for me at dinner, and as I was obsessively chewing my food worrying about choking, I would take at least twice as long as everyone else and the embarrassment made the anxiety worse, especially when my siblings would get impatient wanting to leave the table. In general, try to make it so no pressure is attached to eating.
Hopefully this gets better on its own (it did for me after about 3 months) but you're right to be seeking help - I know how horrible this is for him and you, good luck to you both!