Is he at home all the time, or does he attend nursery/preschool? If he does, have you spoken with the staff there to see if he's the same?
Does he get to socialise much with other children? If so, how is with them?
Typically, at his age, you'd expect a child to be able to hold a pen pencil almost properly and be able to copy a circle. Is he able to do this, and is it just the motivation to draw anything that's missing?
What about trying to gauge his interest with different ways of drawing such as with paints? Or outside with chalk on a nice, sunny day?
Being able to discriminate letters and numbers, and reading books, really isn't much of a concern before he starts school to be honest. But he should ideally be able to sit and attend to you reading him a short story. And should know how to turn the pages of a book properly, and that it goes from front to back. Instead of proper stories just now, why not get yourselves to the library and look for some picture books or sensory/interactive books? I think at this stage, the focus should be on getting him to turn the pages, and listen to what you're saying about the book, and encouraging him to point things out to you in the pictures etc rather than sitting looking at a page that is mostly text.
His playing sounds great and the fact he can build rockets etc out of Lego is fantastic! How is his turntaking? And his speech?
Right now, the focus should be on play. At this young age, that's where children learn most.
If you do have concerns about his behaviour/development, it's always great to have a professional chat with someone e.g. the health visitor who can come out and observe him.