Is it a preschool for a few hours (then time at home) or a day care school where he spends the entire day?
My two boys (5 and 2) are also very academically advanced but have thrived in play-based environments. When my older boy was 3, he starting wanting more, so we would play reading and math games together after school (his school was finished at lunch). When my 2yo turns three in a few months, we'll start the same, though with the help of our nanny as I've gone back to work.
What you're doing at home is obviously working, so I'd just continue. I find personally that time together reading or playing games is more effective for learning than computer-based options, but there is a place for those as well.
I haven't heard good things about ABCMouse, but Starfall (the free version even) is wonderful. Dreambox is excellent as well, and while Reading Eggs was never a good fit, Math Seeds was something that my son enjoyed for a short time.
