There is a connection between the middle ear and the nose. This is known as the Eustachian Tube. When it functions properly, it equalizes the air pressure behind the eardrum to that of the atmospheric pressure (through the nose).
The Eustachian tube is wider, shorter and more horizontal in a child (than in an adult, where it becomes narrower, longer and more vertical as we grow). Liquids such as milk can easily enter the middle ear especially if your child is drinking milk while lying flat on the bed. Try and raise his head a bit while feeding.
Frequent colds may be a result of hypertrophy (increase in size) of the Adenoid tissue behind the nose, which normally occurs between the ages of 3 and 7. If he has developed snoring or mouth breathing, please ask for a lateral x-ray of the skull to rule out adenoid hypertrophy.
There can be a number of other reasons for poor ventilation of ears. An eardrum perforation means that nature is trying to compensate for a poorly functioning Eustachian tube.
Remember that the middle ear is always wet due to the presence of mucus secreting glands and hence if your son has developed a permanent eardrum perforation, his affected ear will not dry up completely and pick up infections easily.
Anti-allergic medication and mucolytics along with steam inhalation (under strict adult supervision) are the simplest remedies available.
Every time infections develop in an ear, toxins present in pus can diffuse into the inner ear and permanently damage the hearing, In rare cases, it can also affect the brain.
More complex treatment, if required, is possible under the care of an ENT specialist.