I did not have to pose him. Most of the he was just enjoying himself, and I just observe and shoot. Sometimes when he is really happy I just call out his name to catch his attention before I shoot.
Most of the expressions are his reaction to things that are happenning around him, for example, in #4 he was laughing at my father-in-law who was having a good time on the swing in the playground. In #5 he was playing hide and seek with my wife.
A lot of this is based on instinct as well as knowing my son enough to anticipate his reactions to things around him.