In infants and children this is a strong indicator for congestion. As the sinus tissues are inflamed and clogged, the glands produce more saliva. This leads to more swallowing and an effort to clear the passageways.
This type of congestion may not always produce a nasal discharge. If it is very thick, it can just sit there and cause problems. Also, it is possible for fluid to be trapped in the inner ear without there being an inner ear infection. If this is the case, the fluid will be clear and not easily seen except by an ENT.
I would try some home remedies for congestion and trouble shoot his diet for foods that are mucus producing. It is likely that he has a favorite food which he craves on a daily basis that is the culprit.
I would also take a critical look at things like his balance, coordination and enunciation. All of these can be affected by this type of congestion as it affects the inner ear and hearing.
It is also possible that he had a problem in the past that has since been resolved and left him with the habit of simply allowing the excess to drip rather than swallowing it. You describe a constant condition. He may likely have become so used to it that even after the initial stimulus went away, he was left with a maladaptive habit.
At any rate, I'd be curious to know what an ENT had to say about the structure of the sinus and its development in your little guy.