I am not a kid but I have a post nasal drip which bugs me and has done since I was a kid. I work in a dental surgery and we work closely with ENT's and naturopaths and what we have found is that most kids who have breathing/ nasal problems are latcose sensitive. Maybe try taking your child off Dairy for a week or so and see how you go maybe? I know I have basically no posta nasal drip when I havn't had any dairy.

It may work and perhaps even save a trip to the ENT?