M.F.
I would definitely pursue it. Since an ear infection was ruled out, you might see about a vision screen. You'd need to see a pediatric opthamologist. My daughter goes to Dr. Alan Norman with Pediatric Eye Specialists. They have offices in Arlington and in Hurst.
It may be that that little bit of fluid IS enough to cause him to stumble, even though it's not an infection. Maybe allergy medicine, or something like Mucinex would help relieve/drain some of that fluid.
I will say that my son has also gone through these "clumsy" phases -- literally just walking across the floor and falling down, or bumping into things. Not sure what it is except maybe a growth spurt. He'll grow a couple inches in what seems like overnight, and I guess it takes a few days for his disproportionate little body to get used to itself again.
And the head bumps were simply a regular part of life when he was the age your son is. I babysat a boy the same age as my son and I swear as soon as one forehead bruise went away, one of them would fall/crash into something and get another one. It's funny to look back at pictures from that year. So many show them sporting bumps and scrapes on their heads. When you think of how those little guys are proportioned, it would be like us walking around all day in clown shoes, with a bowling ball for a head. It's amazing they're even upright :). My boys would run into chairs, walls, toys, tables, each other, you name it...trip over nothing at all, or their own feet, or step on toys and fall. I spent all day trying to clear walkways, or cushion play areas. However, my son is a pretty agile kid now. I have caught him with one foot in each of our fisher price little people school buses, using them as "skis." Really, I don't know how we've avoided broken bones with all of those antics!