I just ran into this last week when I had my former nanny kids for a visit. Kid fit fine in the Literider in the second row; requested to move to the back and the fit was all wrong. I made him give me the booster and fit was fine
(He's 8.5!) Anyways, yes, if it's sitting across the thighs and not touching the hips, it's too low.
Weird suggestion, but what about using one of the boosters where we generally *don't* like the fit - I'm thinking Evenflo Big Kid here, not AOE in booster mode - and seeing if it improves it on him? Some of the boosters that do SUCH a great job positioning the belt on 4, 5, and 6 year olds are going to pull it too low on 8 & 9 year olds who aren't *quite* ready for seatbelt only. Ambassador is a good suggestion, maybe the Cruz (?), maybe the Top Side? I think all of those would be good options to try, although it's harder to justify the purchase-return schtick on $15 backless boosters, lol. Do you have a Parkway you could try backless? (IME, the Monterey backless is actually pretty bad about this, because of all of the distance kiddo gains to slide back once the back is removed. Does he still fit in the HBB portion of the Monterey?)
The other option is to put him in a seating position where he *doesn't* almost 5-step and see if that improves the fit in his current booster.