Hi J.-
It could be that there is persistent fluid in his estuation tubes that's not draining properly. My son had that problem (his tubes are smaller than normal). Our pediatrician had him on a course on low dose antibiotics for 6 months, from age 12-18 months first (12 years ago) to see if that would dry up the fluid before sending us to an ENT. Ultimately he needed ear tubes twice. Once at 20 months and again at 4.5 years old.
Our son spoke normally, but after the first set of tubes, his vocabulary exploded! The ENT and audiologist said that he had most likely been reading lips and learned to speak that way (his hearing test results at 18 months showed that he was deaf).
If you do end up needing an ENT, ours was wonderful! He's in Fremont and was great with us this first time and awesome with our son the second time. He was very thorough making sure we understood everything and all our questions and concerns were addressed. The second time we were more relaxed and the doctor focused on our son rather than us with the discussion and spoke to him in terms that a 4 year old could understand.
Advantages to the ear tubes were, no issues with equalizing pressure while flying in airplanes and we had NO ear infections while they were in (takes about a year or two before they are pushed out by growth in the area). Downsides, you can't swim in lakes or rivers, and you can't go under water more than a few inches in a pool or tub without earplugs.
Good Luck!