Is it corn allergy or corn intolerance, or could there be something else going on such as a gluten issue?

You really need to see an allergist ASAP. This is not something to mess around with. My son was just diagnosed with a corn allergy and even though his only symptoms have been eczema and some gut issues, we still left the appointment with a prescription for an Epi-Pen. Given your son's severe reactions (throwing up, etc.), further exposure could lead to an anaphylactic reaction. I ume that your husband and your son's grandparents wouldn't want to feed him something that would kill him, so maybe they would listen if you could get a definitive diagnosis. Good luck to you.