Hi Sabrina,
Post nasal drip is awful! Basically any one of the meds listed above will work, and I find that drinking a lot of water helps, as if flushes and thins the mucous. Also, a neti-pot works wonders, and it's natural and provides immediate relief. I talked with a pharmicist who said her 8 year old son uses one. It's a little strange at first, but you get used to it. I hope your son gets some relief!

Best Wishes,
Zen