A tightness in the throat along
with gagging is usually a sign of upper respiratory infection along
with post nasal drip. It can also be due to allergic sinusitis too. How old is your son? Tightness in throat along with gagging can also be due to stress, anxiety and as a part of panic attack. Yes, GERD too is another possibility. You can give ranitidine and see if this helps. I would suggest an ENT examination at the time when your son is having these symptoms.
Hope this helps.