The packaging may have indicated that it was the correct dose for Minion. But like humans, some animals are more sensitive to medication than others. If the Advantage was applied on bare skin (versus through the cat hair), he may have taken in more than usual.

You might want to try a dose of Frontline to see if he has the same problem. Otherwise, you are right to consider decreasing the dose (especially if he is an indoor only cat).

Good luck