I have a 9-year-old who is probably gifted and enjoys mathematics. I'm not sure I have much helpful advice for you, though, because her educational situation is quite different from your ds's. She's unschooled, and just leads her own education, having free range over whatever resources she can find, or whatever we happen to have around. She is almost never systematic about working in a prescribed sequence, or completing an exercise or program before moving on. Led by interest she has dabbled in areas way out of the "normal" order. For the past six months or so she's done hardly any math bookwork, and that's been fine. She's grazed a bit through Ed Zaccarro's "Challenge Math" book, dabbling in probability and statistics, and and reviewing basic K-7 arithmetic ... but often goes a month or so without touching any of it. She started an art class a month ago, though, and they're doing work with spirals and mandalas, and she's got hooked on the math-art connection. She spends hours watching and then playing around with Vi Hart videos, working out Fibonacci tricks, binary trees, making hexaflexagons, and so on.
So my dd's math education has been largely interest-led, full of rabbit trails and wormholes. Same with my older kids, who all entered school at least a year or two ahead in math despite the relative lack of systematic bookwork or curriculum. Not at all Montessori-like, at least from what I know. (I honestly don't know what Montessori math looks like beyond the 3rd or 4th grade level, so I'm not sure what to suggest.)
We did use Singapore Primary Math in a loose way during the early years. Depending on what level your ds is at and what learning style he prefers, it might be suited to self-paced independent study use in the classroom. It's big on concepts, and big on mental math, and presents fairly complex word problems at early stages. It's low on repetition, and my kids managed to work through it with very little adult guidance. It only goes up to a 6th/7th grade level, though -- and it doesn't include much that's outside the [narrow] scope of a traditional K-7 curriculum.
Hopefully the book Geofizz suggested will be more help to you.