My K DS found the Beast Academy books I have for him for summer math and started pouring through them. He begged me for each one after he was done with the other. He read 3A, 3B, 3C and part of 3D in under 2 weeks. I know he isn't getting all the concepts this way but he's asking some really interesting questions. He literally spent 5 minutes jumping up and down and screaming "I love this one!" when he got to 3B about multiplication. This is a really reserved, laid back kid.
He got more excited about this than the donuts I brought home last night.
We'll see how he does when we sit down and have to do the hard work that goes along with the books. But I just thought I'd add our experience and say thanks to everyone that recommended these books to me. I'm looking forward to a fun summer of math work.
For the school year we're doing Dreambox, mostly for double digit addition/subtraction practice. It's been a good program. We're just now moving to 3rd grade content so I'm not familiar with the level your son would be working at, a year ahead of what we've been doing.