I've heard that dogs have sweat glands in their mouth and also their paws, maybe licking for the salt in the sweat,and/ or to clean them.

Unless there is something wrong with your dog's paws, I think he does it because it's soothing and relaxing to him. My dogs do the same thing. 

If your dog is white and the paws have turned a pinkish colour it is a sign that the dog isn't digesting his/her food properly.  It is a precursor to the dog having food allergies.

Three of the highest allergens in dog food are, corn, wheat and chicken.

Many dogs who incessantly lick their paws have allergies.  I am not saying your dog does, it is just a possibility.

grooming.

Please click on the link below and read the article, it may help you in the right direction to take.  

http://www.ehow.com/facts_5167100_do-dogs-lick-paws-constantly.html 

Pain

It is always a good idea to inspect the dog's paws for anything abnormal. Dogs generally tend to lick when something is bothering them. Check for thorns, broken nails, swelling, open wounds, scratches and insect bites.

Allergies

The problem could be the food you are feeding your dog or the pesticides or fertilizers used on your yard. Skin allergies frequently affect a dog's paws.

Anxiety

When exposed to highly stressful situations, some dogs may resort to licking and even chewing their paws. This is often seen in cases of obsessive disorders or separation anxiety.

Boredom

A dog left alone all day in a crate may become frustrated and resort to licking its paws to keep itself occupied. This can be seen in high energy dogs that require lots of daily exercise and stimulation.

Considerations

Left untreated, paw licking may lead to a condition where the paws develop unsightly ulcers known as lick granulomas. To prevent this and find the underlying cause of the excessive licking, consult a veterinarian.

Some dogs do it out of boredom.

He does have food and water right?