Hiya

Not sure how much help I can be - but I'm an adult sufferer of ARFID and I've had it all my life - though was only diagnosed about 10 years ago.
So, what you've described with your son DOES sound as though it could be ARFID. I know you don't need my life story (will keep it brief) I developed emetophobia when I was a toddler and that was what brought on my eating disorder.
This manifested as me constantly gagging/choking and fearing food. Couldn't eat what my parents did and I was restricted to the same small group of foods (often dry/crunchy) every day. I'd have the same thing for breakfast/lunch/tea every day and the amounts I ate would vary depending on how anxious I felt...
The single most important thing I'd say to you is - whatever he's eating, however he's eating it - just let him for now. With ARFID any nutrition, from any source, is better than nothing. Completely understand your concerns around calories and vitamins, though, as it can be an issue (it is still for me now). There are tasteless/odourless vitamin powders you can buy that can be sprinkled and dissolved into drinks - I've seen a brand called Nutrigen that some parents of kids with ARFID recommend.
https://www.arfidawarenessuk.org/ is a good source of info and support - and a good place to start before you see the GP, then you can be armed. with info.
Are you on instagram or anything? There is an account called eff_and_arfid, and it's a mum who posts about her young daughter who has the condition - she runs a support account with two other mums, who have ARFID kids who are about your son's age, too - they post loads of useful info and support.
I'll stop rambling, but just to say you and your son are not alone. If nothing else, I completely understand... <3