Though Aiden smiles and laughs all the time, his ability to sense an approaching camera is sharp, and he immediately changes his expression. Thus, the best image of him smiling was snapped while Clyde wiped Aiden's mouth (and Aiden was distracted).
Clyde and Aiden.
Exhibit A:
Clyde Tyler at 4 months (August 1978).
Exhibit B:
Clyde Aiden at 4 months (July 2011).