DS has a
Casio FX-991ES Plus which he's been very happy with (and I've been very impressed by - calculators have come a long way :-) In particular, it has the feature your DS wants of displaying the calculation in full and its answer.
IMO, at 6 your DS is probably ready not to be confused by a scientific calculator and you might as well get him one; it'll last longer than a more basic one.
On the other hand, I have friends who swear by graphics calculators for mathy kids, and I've never quite seen the point there - it's not clear to me that they have any advantage over the combination of a scientific calculator plus Geogebra for when you want to plot things. I could be missing something there, though.
