I can't comment on the specifics, but I do want to comment, in case you didn't know it, that the fixed order of US high school maths courses is peculiar (and I do mean peculiar :-) to the US. In the UK, for example, the material of these courses is taught in a much more interleaved way. Looking at the equivalent course summaries in ALEKS it looks as though Algebra 1 followed by Algebra 2 would be fine; there might be the odd thing that needed to be filled in, but I don't see any very major issues.
That said, I think your idea that it would be good for your son to do a variety of kinds of maths is a sound one. One thing you might consider, perhaps, is having your son do some geometry but less than a full year course of geometry - maybe something basically problem-oriented/recreational, since your son obviously has time in hand and that kind of thing is great for developing mathematical thinking? There are some super stand-alone materials out there, although I don't know which ones would be at the right level for your son. We've got the Art of Problem Solving geometry book, but he may be beyond that? I've also just bought several geometry books from the UK Mathematical Trust, here:
http://www.mathcomp.leeds.ac.uk/publications/index.php?type=Bookwhich look super (although they are mostly beyond my DS for now, so we haven't used them seriously yet). The various maths challenges and olympiads are a rich source of geometry problems, too. And then there's the rest of maths... but this is long enough so I'll stop.
