I think you may like the Zaccaro book. It covers lots of different concepts and you can do whatever your son is interested in. It covers probability, negative number, square roots, circumference and area, decimals, logic, percents, fractions, ... It has no geometry though. It's word problem oriented and has 4 levels of difficulty.
Fred is funny. It's a nice reading and you get to laugh a lot. It introduces new concepts but it may be going little too fast. You know not really enough practice.
You could also get the next few Singapore books and do whatever part he is interested in. Like I said DS does what he wants to and once I think he knows everything from a given level (regardless of what he did or didn't do in the workbooks) I give him the placement test just to make sure. You could also try the Singapore Challenging problems.
I think you can let your son use the calculator. Concepts are much more interesting than pure counting. Let's face it how many adults really use long division? Of course he will have to learn it one day but it shouldn't keep him from enjoying the more interesting topics. We usually help our son to do the calculations on the paper, sometimes it's us writing it down and commenting on how it's done and asking him simple questions. That way he is exposed to it and will learn it when he is ready. I think it worked quite well for the multi digit multiplication, long division will probably take more time.
