No.
There are certain tax benefits that a person is eligible for when they are a citizen or permanent resident (green card holder) that are more generous than they are eligible for when they are a non-resident alien or resident alien. In the past it was possible to amend a return to retroactively claim those benefits.
However, due to a law passed a couple of years ago, that is no longer allowed. He only qualifies as a citizen on any tax return that is filed after the awarding of citizenship, and only if the return is "timely filed." That means that if you have not filed a 2016 tax return yet and you have an approved extension, you could file the 2016 return with him as a citizen. But you can't amend earlier years. And if you already filed your 2016 return, you can't amend it either.
