Don't worry that it seems impossible to imagine them reading long books by themselves - for a start, five is very young, but also (in my experience - obviously there is huge variation) their reading ability can suddenly take off when something 'clicks'.
With DS, for example, at five he was also reading one-sentence-a-page readers from school, when I could persuade him to read at all - he appeared to have no interest at all. But I realised a few months later that was because the school readers were deadly boring, as when I tried something a bit more fun (Dr Seuss) and then something that tied in with his actual interests (some Scooby Doo early readers) he was much more enthusiastic and actually made an effort.
Then, not long after he turned six, I started reading him Harry Potter, one chapter a night at bedtime, and he was so gripped that by half-way through book two, he grabbed the book and started reading it himself because one chapter a day wasn't enough. Of course HP was light years ahead of the reading level of books he was given at school, but because he was interested, he was willing to really, really try. DD was similar, but took a little longer, and we had an excruciating few months of Rainbow Fairy books before she moved on to 'real' books.
So I think you are doing exactly the right thing - reading to him, & finding stuff he'll enjoy. I would say, whatever you do, don't make a big thing of it or start pushing him to read more advanced books. But maybe if in a year or two's time you know he's got the basics, and there is a book/series you are reading to him that he is really keen on (almost addicted to) you could try rationing it out: say you can't read more than a chapter at a time or whatever, but he's welcome to read more himself... And just see what happens.