I was searching for an antonym to 'astute' and an antonym derived from the word itself seemed perfect in the context. Does such a word exist?
"Un-" is a perfectly legitimate prefix, and there's nothing to stop you from prepending it to "astute". Though not frequently, it is used, and mostly, it appears, in fairly literate works, vs magazines and popular fiction. "Inastute" is also used, though less so.
Perhaps we dumb down the antonym in this case so that the subject understands what we mean about them? (kidding)
Frankly, I think "OBTUSE" is the antonym I prefer.
OBTUSE adjective 1. not quick or alert in perception, feeling, or intellect; not sensitive or observant; dull.
Consider, dense and thickheaded
dense: slow to understand : stupid, thickheaded was too dense to get the joke
thickheaded: sluggish and obtuse of mind