Your title references "pre-algebra" readiness but your post seem to focus on "algebra" readiness so I'll touch on both. Keeping in mind that individual districts may have different criteria, I still think that a good rule of thumb for "pre-algebra" readiness is a deep understanding of fractions and the ability to manipulate same (multiply, divide, convert) easily as well as developed abstract thinking ability. Any decent math workbook that features fraction problems should be sufficient.
As for "algebra" readiness, I think the best approach is to master a solid pre-algebra curriculum. Is it that your DS will be skipping pre-algebra? Many districts have their math curriculum on-line or simply look through their textbook. DS took a national algebra readiness exam last year at the conclusion of his pre-algebra course. There was no preparation involved in the same way that you wouldn't study for the MAP assessments.