My child responded to animal stories well at that age - try making up a story about 2 monkeys on a tree and 3 more coming to play with them - then ask him how many monkeys are now playing on the tree. Vary this with a different animal or bird story for a few days. Make it very natural and ensure that he does not notice that there is any math involved here! If he gets it right every time, then, it is just a phase. Good luck.
PS: Animal story problems work very well for subtraction concepts as well.
