Your story is eerily familiar...
You may just be dealing with someone who needs that stimulation that they are only going to get from exploring algebra, physics, astrophysics, calculus etc. etc. and all the stuff that the school dist. does is just back-fill.
Your story sounds eerily familiar too! I had a zaccarro book and he did indeed like the work on astronomy problems, but sort of petered out after that. I have just recently gotten another book on pre-algebra, one on algebra and one on 'solving problems in astronomy' to see what would appeal, and to have something more substantial to hand in for the 'work at home' aspect of this assessment.
Well, the pre-algebra was ok for a few minutes, but quickly became obvious that he knew all there was or picked it up so fast it was like he knew it already...ok, onto the algebra book.
Granted this thing starts with pascal's triangle, which he's checked out before, and math with exponents, substituting for variables/etc., (easy stuff) but we are 1/4th of the way through the book in a couple of days and not finding anything to have him even pause to think about it before he's like 'ok', I get it, does a few problems and gets a bit bored with the repetition aspect....
This morning I rolled out the astronomy book and the ideas about ellipses and orbits was finally something a bit 'thicker' that he could sink his teeth into. We just chatted about that...
He just came into the room with his ski goggles on and his microscope, so I think I better end this post, but you get the idea....oddly HATES practicing with these concepts, but he seems to pick them up VERY fast.
Hopefully some of the work he is doing will hit the mark with the assessment folks...