What does your ped say? I trust you've asked him/her about this? In my not-so-medical opinion, this could be anything from a texture issue (maybe a sensory integration problem) to a physiological issue. My nephew had an issue at birth where his trachea was fused to his esophagus, which required immediate surgery. Now, he has some issues swallowing, and can asperate liquids into his lungs. He's gone through a few swallow studies to see if he's swallowing normally. The swallow studies aren't fun, but your son may need one to rule out any issues with his innerworkings.
I've known people whose kids had sensory integration issues that just refused to eat solids, and ate baby food for about 2 years. It took some therapy to get them to eat.
(NOTE! I just realized you said your ped passes this off. I'd definitely get a second opinion!)