Since it sounds as though challenge, more than linear progress, is what you're looking for, I'd suggest
Alcumus. (I know, I usually do suggest Alcumus - but really, it's very good.) It is basically a lot of problems, often quite brain-teaser-y, with a topic tracker to get the level right for the student and some videos etc. as backup; if you wanted systematic teaching materials you'd probably want to buy a copy of their Intro to Algebra book as well, but if you're able to be around and help on demand you can do without that (we have the book, but hardly ever consult it; generally DS does a few problems on the bus).
