If he'd said something before R1, it would have been embarrassing for him to lose to that kid, and embarrassing for the kid to be the one to have ended Roddick's career (probably only to lose in the next match). The kid is American so Roddick doesn't want to undermine him.
He probably would have announced it in his post-match press conference and the USTA would've arranged some time before one of the 7 pm matches later in the tournament for Roddick to come out and wave goodbye to the crowd (Remember how Sampras came out?).
This. He didn't want to announce ahead of playing a countryman. Fully understandable. It seems a long shot that he will run into another.