My DS7 was similar in that age too. He taught himself the alphabets and was obsessed with them. He would say them forward and backward before he was three. He was able to spell and read simple books around that age too. However, he's more fascinated with the words than reading a book. He loves word games such as word search and Rebus. But he doesn't like to read books. We get books from library all the time for DS8 and him. DS8 reads pretty much everything (fiction or nonfiction) but DS7 would only read books that he's interested like riddles or jokes.
I'd say have your DS keep reading different books. He'll learn not only words but other things that books can teach him. Please also keep in mind that your DS maybe able to read words, but you'll need to know how much he understands it. Comprehension is a very important part of reading. My DS7 was brought down one level in K year by his teacher to enhance his comprehension before moving on to the higher levels.
