I had anticipated him skipping 6th grade math and taking PreAlgebra this year to be ready to take Algebra as a 6th Grader. I still wonder if that's a better idea than skipping PreAlgebra and taking two years for Algebra???
I don't see how that would be better; surely prealgebra will both start and finish at a lower level than the first year of the offered algebra course will, so you'd be holding him back, looking only one year ahead - and a good challenge next year is a better aim than a good fit the year after, because easier to judge.
If he's fine with the algebra, you might at some point want to argue for him to move sideways into one-year algebra next year. If he isn't going to finish the algebra course in one year, then unless there are extreme cohort issues, algebra over two years seems better than something less than algebra for a year and then algebra for a year, doesn't it?