Ryan has naturally curly hair, so he tends to keep it cut pretty short. This allows his curls to provide body to the hair without overpowering and going wild. The style he wears right now is cut close on the sides and back with about 1 to 2 inches left on top. A small amount of pomade on the fingers and twist a few locks up to stand out from the head is the way this look is produced.
Here are some pictures of Ryan's hairstyles

