We get those spurts of energy too. Some days DS5 wants to go on and on and on... and then for a week or so he won't be too interested.
I was going to have him take the 1B test because originally I was thinking that it could go either way on starting 1B or 2A. Unfortunately he's not very not interested in actually doing the test for us, so we need to think about our strategy a little more. He doesn't have a problem doing a worksheet or taking a test for other teachers or whatnot.
It might help if we get it a little more structured than our informal "here is what I remember about insert-subject-here" lessons we've been having. I took a look at the sample pages that they had online, and I think that's he'd be perfectly comfortable starting with 2A. If I don't do that, I definitely need to get some sort of textbook because he wants to do geometry and I'm afraid I'm at the limits of what I remember!
JB