Our doctor is now recommending not using the OTC cough remedies and instead making a mixture of equal parts lemonade and honey, warming it slightly (like the temp. of hot chocolate) and then swigging all at once. What I did last time was make up about a half cup mixture in a measuring cup and kept it in the refrigerator (for when I was too sleepy in the middle of the night to mix and measure!) Then I would just warm it up as I needed it, pour a bit into one of those medicine cups that come with OTC liquid medicine and give it to my kids when they needed it. Usually about a teaspoon at a time.
My daughter is like your son in that her coughing fits can lead to gagging and vomiting--not fun! The lemonade/honey mixture really seemed to work for us last time!
P.S. Be sure you buy real lemonade and not a powder mix or a juice drink that may have other things added, like extra sugars. I just bought the Simply Lemonade brand and figured that worked best!