So this is to supplement what he's getting in school, which sounds like it will already be somewhat accelerated, right?

I think my recommendation would be to try Beast Academy first and go to Singapore if your child doesn't like it. Beast Academy is much more presented in a way that many kids will find to be fun and engaging, whereas Singapore, while excellent, is textbook math. I've found that even my rather mathy PG child got bored of textbook math on top of school math, even when the school math was a couple grade levels too low. DS6 loves Beast Academy (though I think he's only now approaching the "right level" for 3A) and Dreambox; DD10 has always been just a year ahead of Beast Academy but likes Life of Fred, Dreambox, and "math puzzle" workbooks. 32