Bad parenting?

Willow
Maybe I'm sticking my nose in where is doesn't belong but I'm incredibly pissed off at my stepson's mother. 
He's 4, turning 5 in November and can't read. And I don't mean he can't read books, I mean he doesn't even know any letters of the alphabet, can't write any of them and can't even write his own name. 
Me and my SO only get to see him every other weekend, so what we can do to teach him is limited, and when we talked to his mother she said that when he starts school that they can teach him how to read. 
I don't think that's right at all. I was learning the basics of how to read when I was 2-3 and I could read simple books by the time I was 4, and when I started school at 5 I could already read things like Dr Seuss and Roald Dahl books. 
I feel like his mother is doing him a disservice by not sitting down with the him and starting to help him with his literacy and I'm worried that he'll be picked on at school if he can't read when he goes there. 
I've gotten him some alphabet wall posters, along with a numbers one, and some beginners writing practice workbooks and when we have him for the weekend we're going to start having short lessons with him to get him started. 
It's more than just not teaching him a skill, love of books and reading is such an important part of life, I was always stuck into books when I was a kid, it seems so unfair that he's missing out on that already. 
Am I right to be teaching him if his mother won't? And is she failing him in a way by expecting school to handle teaching him instead of doing it herself?