It sounds like your son is one of the lucky boys who can learn symbologically at an early age without damaging his neurological development--I'd agree that, he is doing really well! Congratulations :).
Please consider not "pushing" him, though. Really, if a parent has a chance to stop and step back from the messages of society, it becomes obvious that children are naturally eager learners ... and that we even use the terms "push" and "compulsory schooling," as if they were natural to how children are, should tell us something is wrong with our picture ... if we are having to compel them to learn, the system and its assumptions for the children are deeply in error. Children LOVE to learn. It's what they deeply want and utterly NEED to do ... if they are fighting it, there is a reason. Kids want to be loved, and they want to grow up into healthy adult people ...
... and not that I haven't (and don't) sometimes force my kids to learn a thing (a behavior, mostly) ... but I wince, because I know that in a healthier society, a healthier family, and a healthier ME, they would be learning by exploration and by imitation (of my wonderful self, you know, ha ha ha *groan*), and there would be no "pushing" of adult agendas. Kids want to be grown up--they push themselves ;)! (Sometimes they push themselves too hard--I have to hold my kids back in some arenas, when they hear society's pressures or get caught up in the fun of something they find enjoyable but that is not safe for them or those around them.)
I have a bunch of related stuff to say about neurological development and the horrors of the assumptions behind standard American education ;) ... but again, it sounds like you and your little guy are among the lucky, so I won't bore you with all that ;).
My third is learning to write her name ... it is so darn cute :)!! I'm interested to see how form drawing goes for her, because she makes everything curve together ... no sharp corners ... but that's a topic for another day :).