Awww, bless his heart! Buy him a shake!
They're not as old as they think they are. :) When my kids do this, I am always reminded of their baby/toddler/preschool days, when they would fall asleep in the car on the way home from the Children's Museum or the zoo.
Ah, yes, we've seen that a bit here, too! Makes me kind of long for those baby nap days...