Do research ARFID..
Im an adult with ARFID, raised with the 'eat it or go hungry' and the 'if you don't eat it it will be served to you at the next meal and the next until it goes visibly mouldy' attitude.
So I am an adult with ARFID now.
Things that help me now that your kiddo can't yet do - being able to buy my own food, being able to prep my own food.
Things that help now that can help your kid:
Offer all the food for a meal on the table at once - put it in seperate dishes or on separate plates if thats necessary, or on divided plates.
Make sure that there are several things he does like - and do not fuck about with that - if he says straight cut McCain chips, don't put out Morrisons own brand and make out they're the same, they are not to him and he knows.
Put a couple of new options out too, it can help to go with similar things to his current safe foods but DON'T push this too far or you can go the other way and put him off his safe foods.
He eats what he wants from the selection available.
You say absolutely fucking nothing, not a peep, not a whisper, zip it.. nada about it.
So no encouraging to try, no 'if you don't eat x you can't have y' (after all if you determined both X and Y were appropriate food nutritionally, why does it matter if he has one and not the other if the original plan was he have both?)... no 'SEEEEE it WAS nice after all'... absolutely nothing at all.
At the end of the meal, if he is still eating, clean up everyone elses stuff, go off and do whatever it is you were all doing - he can finish up or he can choose to leave it at the same time you all leave the table, but you won't all sit around waiting for him (because this puts pressure on him, not because you're punishing him to eat faster).
Do not try to hide foods in other foods - this is a really quick way to put someone off a safe food.
Do not comment - at all. I really can't emphasise this enough, even if your comments are intended kindly, to be encouraging, to praise him for trying etc, they are not going to help.
When someone with ARFID tries something new and doesn't gag or instantly refuse it...
And someone helpfully says 'see, its not so bad/see, its nice really'... what the person with the food issue hears is this:
'You're imagining these issues, you're silly and your opinion doesn't matter, you're wrong, look how wrong you were...'
Very quickly even a 'well done' or 'you're awesome for trying that' can come carry the same meaning.
See if he'd be interested in doing a Super Taster test - its just a paper strip you touch to your tongue, super tasters will taste it really strongly (bitter, like ear wax!), normal tasters will taste it fairly faintly and non-tasters will barely detect a thing, if at all.
Im a super taster... in a family of non-tasters. The things I was detecting in foods they hadn't a clue were there!
For most ARFID sufferers, both texture and taste are an issue - I've got issues with food being too dry too, likely due to a hiatus hernia, ditto with things being in large pieces or taking large bites (which I was encouraged to do as a kid to get down things I didn't like quickly). We tend to be very quick to identify how something will feel and taste simply by looking at it -we're not always right in that, but that link is very very strong for us!