As someone who has a strong gag reflex I can really sympathise. I think I would hide it in something like honey and tell him that the honey will help soothe his throat (or whatever the symptom is). If he doesn't know there medicine is in there it will help him get it in. Otherwise if he can at least get the syrup down chase it with water or milk to get the taste out of his mouth. I could only swallow tablets as an adult and only this year have learned to take them without shoving them down my own throat.