I had that with one of my daughters. She refused everything but mashed or pur?ed food. It turned out to be a whim she had with me - she ate perfectly normally at the creche! However, if youi don't see it it's hard to believe. She eventually gave herself a way one day when she grabbed her sisters corn on the cob and proceeded to wolf it!
Having said that there are some very real causes for this. Have you mentioned it to your doctor? Your son may have sensory issues with textures (among other possibilities) ... I would have that checked just in case.