Either he has something stuck in it, or it is infected. Walking outside sometimes irritates the paws. However, don't worry. It is definitely not harmful in any way. It is a habit that can be broken.
If it licks its paws more than usually, it might have a sore paw, an injury, an insect bite, a splinter (sometimes it's glass and it can be difficult to spot it).
Dogs sometimes lick their paws because they have allergies and their paws itch. This is usually the case when the dog is also biting at their paws. Dogs are able to take the medicine Benedryl. You...