Both of my boys (2 and 4) have allergies and post nasal drip. When they are sick, or have just gotten over being sick, the smell is a lot worse than normal.
There isnt really a lot you can do about it either. I buy cream of tartar powder and I mix about a tsp with a little water in a medicine cup and I have my older son gargle it twice a day. He's old enough to spit it out, and hes also old enough to realize that it helps him, and he'll ask for things that help him these days. It's a really salty powder- I dont know what it does or why it works, but it was something my mom told me to try and I was willing to do anything.
For my son, it really seems to soothe his throat and thin the mucous.
I would also keep a close eye on your child's ears. Ear infections can be a lot worse in kids who have PND. I've always been fond of the "smell test". If I smell ANY kind of funk when I sniff my kid's ears, its time to go to the doctor. Sounds gross, but a mom's job is.