Your son sounds a lot like my younger daughter (she will be 5 this month). When she had just turned 4, she became very interested in reading (I agree with you - they tell you when they're ready). What I did was create flash cards with 2 or 3-letter words on them that she could sound out with me, such as hop, on, fat, cat, pop, map, mom, dad, sat, up, etc. Then after she sounded them out, I'd have her re-arrange the flash cards into a sentence, read the sentence to me, and then draw a picture to go with it. For instance, she would make a sentence that said, "Dad sat on the fat cat." She thought this was a great game to play! Once she understood how to sound out little words, we were able to sound out bigger words. By the time she went into Pre-K (at 4 years, 3 months), she was reading simple "I can read!" books. After a month in Pre-K, they moved her into Kinder because she could read sentences, paragraphs, etc, and then answer questions about them, which was way, WAY past what the other kids in Pre-K were doing.
We did the same type of thing with math when she turned 4. I would give her a handful of objects (pennies, little counting bears, buttons) and would say, if I have 2 pennies and you give me 3 more pennies, how many pennies will I have? And she would give me 2 pennies and then 3 more, and count them to make 5 pennies in all. Then I would write the number sentence (2+3=5). Or we would have some bears on a bus, and more bears would get on the bus (4 bears are on the bus, and 2 more bears get on the bus. How many bears are on the bus? - And then write out 4+2=6) We did the reverse with subtraction. So she could do addition and subtraction up to 20 by the time she went into Pre-K, using manipulatives.
I should note that with all of this, I did not push her at all. I let her tell me what she wanted to learn about, and if she would go get the jar of buttons and ask to do math problems with them, then we would do it. If she brought me her word flash cards, we would do that. She thought it was a lot of fun, and if at any point she wanted to stop, we would. As a result, she really learned a lot last summer and really flourished in school this year. She just graduated from Kindergarten last week and LOVES to learn new things! Her big sister is going into 3rd grade and my little one wants to know all about multiplication now since that's what big sister is learning in school. So I guess we will find a way to demonstrate that with our little counting bears, buttons, and pennies this summer!
These are just some ideas that helped us. Maybe they will help you, too. Have fun!