My son is in 5th grade (US) and since he is doing remote learning, we have been doing a lot of topics in pre-algebra just using worksheets. I'd like to start him on a formal middle school curriculum, preferably one that has rigorous fundamentals, similar to the Spivak calculus book I used way back when.
You could consider the series distributed in the United States by singaporemath.com:
- Primary Mathematics for grades 5 and 6
- New Elementary Mathematics for grades 7 and 8.
You can also find later volumes of New Elementary Mathematics for grades 9 and 10 for sale on the internet. Primary Mathematics has three versions adapted for the U.S., while New Elementary Mathematics is not adapted (hence uses metric measures only).
Secondly, for grades 5 to 10, you could consider the recent Australian series ICE-EM Mathematics, now in its third edition, whose authors include a number of PhD mathematicians. It is published by the Cambridge University Press and comes recommended by Fields medalist Terence Tao.
Unlike the Art of Problem Solving series mentioned in the comments, the Singaporean and Australian series are intended for a wider range of abilities (although New Elementary Mathematics is probably for the top 20 to 30% in Singapore schools, which would equate to a smaller percentage in the U.S.)
An alternative to these would be various mid-twentieth century British textbooks aimed at higher achievers, starting from about grade 7 level. One would have to tolerate the fact that they are obviously so old-fashioned, but the level of competence developed in algebra, trigonometry and particularly geometry (in students of a mathematical bent) is without comparison to more recent textbooks.