Allergies! Our now 21 year old was a chronic ear infection kid. He had 3 sets of tubes before he was 2 and his adnoids out. The reason for the several sets of tubes is because when he would build up fluid, it was so thick that it would clog the inside of the tube. The doc couldn't see it when he looked in his ear, it just looked clear. They only found it with a tympanogram, or measuring sound waves in the ear.
I finally switched docs, for a variety of reasons, and happened to find a doc with a subspecialty in allergies. As soon as he said that it all started to make sense. I started to pay attention to not only the outdoor seasonal stuff, but indoor things too. And not just the milk but other foods. Yellow Dye #5 and #6 have been shown to cause allergic responses in those who are sensitive. *I wish I would have thought of a chiropractor at the time.