Unless you just brought him to the US for the first time recently because the other country wouldn't release him for years or you were living outside the US, I find it shocking that you let so many years pass without getting any proof of his citizenship, not even a Social Security number.
This lack of caring about obtaining his documents (unless he immigrated recently) may indicate that you also didn't pay enough attention to detail with his adoption process, which could result in him having to be readopted in the US if something was amiss in the original adoption paperwork. There are some horror stories with adoptive parents who didn't seek any proof of citizenship for their adoptive children until after they turned 16, then in their attempt to obtain a citizenship certificate or passport they were informed that something was wrong in the original adoption process and they need to readopt in the US. But because the children are 16 or older, it's too late for readoption to give them citizenship and their green card is also put in jeopardy (although in this case the green card should be safe if he's had it for over 5 years).
I hope he doesn't turn 16 soon, so you'll have some opportunity to readopt if that is necessary.