I do feel for you.
My god-daughter was about 12 yrs old and she choked on some pizza when we were all out one day. We didn't really think any too much about it. It was horrible as these things are. We were all a bit shocked at the time and upset for her but she seemed ok after and we went to the theatre as planned and kinda forgot about it.
Then about a month later her mum phoned me and said "Jane isn't eating properly." She was worried it could be an eating disorder issue as her daughter is a very keen gymnast and very tall compared to many she competes against. And those leotards are not forgiving.
But my friend also wondered if it was the choking incident.
My friend and her husband had been getting impatient with her meal time was becoming an event. They thought she was attention seeking
We made a list of foods that don't need much chewing, not likely to choke on. Soups, very mushy weetabix, ice-cream, mash potato, scramble egg, rice pudding, angel delight, pureed fruit like weaning babies have etc...
My friend spoke to her daughter ( took her out for a hot chocolate). And Jane was scared of choking again, Really scared. Her mind had seemed to replay it and replay it - and it was now out of control.
And they made a deal that Jane would eat but she could have foods that she wasn't scared of. She looked at the list, added a few more and her diet existed of those foods. It kinda then wasn't an issue and then she gradually, with them hardly noticing returned to eating food as normal. Took time but she's fine now.
I think you let him take some control on what he eats. Think of things together that he'll eat. That will give him confidence. He will build up. Peer groups will help like birthday parties. And hopefully it will gradually fade and just be one of those things.