Khan Academy is one website that comes to mind. At one point DS used
IXL, and found it fun, with lots of different types of exercises which can be picked in any order. We also tried
XtraMath, but didn't find it nearly as good.
Your son might start remembering the facts better in the course of working problems.
Flash Skills problem solving workbooks,
Ed Zaccaro Challenge Math, Singapore Math workbooks and Challenging Word Problems, Life of Fred,
Sunshine Math, and other enrichment may keep him stimulated while cementing math facts and other arithmetic knowledge.
Mathematics Enhancement Programme is a comprehensive online math curriculum for primary grades that is free, and contains both written and online practice. It's quite good in my opinion. Another newer curriculum you might want to explore is Beast Academy, which apparently has lots of fun problem solving built in. Singapore Math is another good one if you're looking for a whole curriculum.
A calculator may be a fun way for your son to explore number relationships and check his answers when out and about. My son loved
this solar-powered one when I gave it to him at around your son's age.
Printable times tables might encourage him to recognize and internalize numeric relationships, making everything glom together faster and better. Our educational consultant mentioned in passing that she's a big fan of these
Wrap-Up Keys as a quick way to demonstrate mastery while avoiding written drill, but I've never seen them in person.
You may also find
this old thread helpful. View the sample Math Mammoth / Maria Miller video for one decent approach to memorizing the times tables.
Regarding raising gifted children, I haven't found any books helpful. I have gotten lots of good information on this site and Hoagie's Gifted, though.