My take on his sudden sensitivity to "the little things" like a playground and a splash pad, etc. is he's showing that he's stressed and finding it difficult to deal with things he would have otherwise been able to with ease. Sort of like how you or I feel when one thing upon another piles up on us after a major life event. Suddenly, you find yourself blowing up over spilt milk.
Has there been a major change in his life recently? A new baby or babysitter? A friend who moved away or a new day care center? Is there tension in the family? Did he see or hear something inappropriate for his emotional age?
I think if you can determine what the major stressor is and remove it, he will go back to his easy-going self and be able to deal with the little stresses again.
