It is not unreasonable to do some executive function testing (including for ADHD), and for that purpose, the NEPSY-II is an appropriate instrument for this age. As to the comparison with older students--yes, the gap may not be quite as big, when comparing to age-mates, but it's still there. The mean age may be seven months older, but I imagine there are multiple children who are far closer in age to him than that in his class.
I don't have direct experience with the DST-J, but what I know of it on paper appears appropriate. It has UK norms, so that is also a small consideration (unless those norms are appropriate for you).
