A child may gag on solid or mushy foods because of preference, in which case he most likely will outgrow this behavior with time. I assume he will also gag on cookies and other treats that children rarely refuse.

If he chews and eats “junk food”, he surely does not have a swallowing problem. On the other hand, one must be concerned with a stricture (narrowing in his esophagus or food pipe), or other swallowing abnormality. This can be diagnosed with a barium swallow study.
LJGMD