The basic skills for reading are, broadly, decoding and comprehension.
Decoding means recognising the letters and knowing the sounds they make, and also understanding how to split words up into individual sounds (phonemic awareness). Then combining these skills to be able to sound out words. It also involves knowing print conventions, like that print runs left to right, how pages turn, the connection between how print is laid out and what it refers to (e.g. the label on a picture), punctuation, etc.
It sounds like he knows alphabet already. You can promote phonemic awareness through songs, poems, rhyming games, alliteration, spoonerisms, as well as sounding out words. Understanding conventions comes through exposure, but you can also point them out specifically. That can also be things like signs and logos.
The other aspect is comprehension, which involves many different things, like vocabulary, grammar, world knowledge, social knowledge, theory of mind, working memory, inferences and implications, etc. Oral language comprehension involves a lot of the same skills, and oral language ability is a predictor of reading success.
With decoding, once they know how, they know how, pretty much. I think this is what people think of when they talk about ‘teaching someone to read’. Most people learn decoding just fine (Possibly relevant, the stereotypical profile for children with ASD is that their decoding is much better than their comprehension). Reading comprehension is more of a life-long journey. The way you talk about your son, I think that maybe you are more concerned about comprehension than decoding, so just know that teaching one doesn’t teach the other.
Just to throw this out there, the fact that he looks at books a lot doesn’t necessarily mean he wants to learn to read. It could be pretend play, where he is imagining himself as a reader. Or he might like the way books look or smell.
Also, to confess my biases. I’m a preschool teacher in Australia. I don’t teach children to read, but Australian schools all teach phonics, from kindergarten, with a few hundred sight words like ‘the’ and ‘most’ thrown in. If there are other ways to teach reading, I don’t know about them. The evidence for phonics is really solid.