Everyone uses phonics to read-- even adults without realising it. Brain imaging scans have determined pretty definitively this is true. Unless you are coming across a word you aren't familiar with, the process of phonics becomes so automatic, no one actually thinks they are decoding anymore once they are a fluent reader (but we all are based on brain imaging research!)
Children who teach themselves to read aren't really learning whole words. My husband taught himself to read before entering school and without his parents even realising he'd done so. I've also know a few children like this. These exceptionally bright children can tackle any new word they come across even at 3/4 because what they've actually done is taught themselves all the linguistic patterns that occur phonetically in the English language. They basically have both good memories and more importantly are also extremely good at pattern recognition / decoding which allows them to intuitively teach themselves all the rules of English phonics. The good memory part enables them memorise books they've heard read aloud and then match up the words to text. The pattern recognition skills allow them figure out decoding rule patterns. These kids read years and years in advance of their age in school. They'd never be able to explain phonics but its obvious its how they are reading very unfamiliar words when you read with them, as they slow down a bit and sound them out quickly (like adults do) based on their own understanding of the rules.
Learning high frequency words before learning the rules to decode them is part of the national curriculum and allows children to tackle more interesting texts from early on in their reading journeys. This isn't separate to a phonics based approach as far as I know but happy to be corrected if some schools don't do this.
OP, no offence but from your description, it doesn't sound like your son has taught himself phonics and can read anything you put in front of him. Therefore, ensuring he's not just guessing what words are but can actually decode digraphs and trigraphs when encountering new words seems to be what is needed as part of the booster sessions. Reinforcing this at home with him in a way that's not excessive should be possible and will ensure he learns to read properly which is key for his future reading comprehension particularly in technical subjects as others have indicated.
I 100% acknowledge that certain children may have quite significant SENs / learning differences that they've developed sophisticated coping mechanisms to tackle given their overall high intelligence.
But again, this doesn't sound like your son OP. At 5, just give him the extra support and see how it goes.