i'm an audiologist and routinely test children's hearing in an ent setting. almost all the tests for your son's age group involve placing a tip into the ear and measuring a response. your level of worry depends on what type of test he had in the past at the pedi's office. if they did a simple tympanogram and he had fluid behind an eardrum, he would fail that test every single time. it measures how well the eardrum moves in response to air pressure put up against the eardrum. if there's fluid, the pressure behind the eardrum is too much and the eardrum doesn't move properly. that's not a big deal and can be taken care of with time, antibiotics, etc.
another much more advanced test is called an otoacoustic emission and this actually tests the hearing organ or cochlea. he would also have difficulty with this test if he had fluid behind an eardrum. it seems crazy that we can see how well your child hears without him doing anything, but this test is great!!
i would most definately NOT recommend waiting until he is three to get this checked out as it's probably something that can be easily treated with antibiotics or tubes. it is rare for a permanent hearing loss to exist in your son's case, but you would want to get it checked out to be sure. also, his speech development may be affected if he has chronic fluid.
best wishes and feel free to contact me should you have more questions.
M.