ETA: Now with pictures!
I had shown him the mirror before, when he was less than a week old, and he'd reacted with a wide-eyed expression I felt sure was awe. Now, at a month old, his reaction was even more positive.
Taking advantage of the quiet alert state he often is in after an afternoon diaper change, I took him into the bathroom to show him the mirror. At first he was looking around at the wall, the ceiling, the window. Then I turned him so that he could see the mirror and held him closer so that I knew his face was in his visual range.
"Look at the baby in the mirror," I told him. "That's you. That's you."
He looked at the reflected baby, and his eyes widened as he met his own gaze. Then he smiled a broad, toothless grin. I brought him just a little closer, so that his outstretched hand touched the glass. "It's you," I said. "The baby in the mirror is you." And I used his name, just to be sure he understood. He smiled, then turned his head towards me, to welcome me into this revelation. I smiled back.
May he always smile when he sees himself in the mirror.
The baby in the mirror is a happy baby.