This exact thing happened to me when I was a similar age, a gulp of water went down the wrong tube and I had a fear of choking/swallowing for ages afterwards. It’ll be psychological rather than physiological. I can tell you what doesn’t help; giving him a hard time about it, making him sit crying in a restaurant for an hour after the rest of the family have finished their meals and he’s had a few mouthfuls, going to doctor/hospital for scans, etc.
Basically I had to learn to swallow food again, I was very bad for a few months and lost quite a lot of weight but as I was a fat little shit that was good anyway. Cold soft stuff was good, yoghurts and icecream with mashed fruit in there, etc. Eventually my parents let me alone and I had to figure it out for myself but it took me about 2 or 3 years to get back to the point where I could eat a meal without clearing my throat between every tiny mouthful.
If he’s anything like I was then he will want to eat, he’s just incredibly scared of swallowing and is overthinking it massively to the point where he has convinced himself he will die if he swallows something incorrectly. So bear that in mind when you’re dealing with this.
Getting angry won’t help, trying to force him won’t help. He’ll get to the point where he’ll get so hungry that he’ll just force himself to eat something even if it’s only a few mouthfuls. But it has to come from him, adding more stress to an already stressful situation will not improve the situation. Obviously if it gets to the point where he’s getting ill over it then you may need to step in again.
I eat quite contentedly now and could do with going on another starvation diet for a few months really.