I swear by our vaporiser. It moistens the air and the vapours really do help DD to breathe through her nose easier.
You can go for some fess to clear out his nose too. It is horrible, but it can be very helpful to them and they get used to it.
If its just a cold, then sadly you just have to wait it out. But I too would see your dr to make sure it is just a cold and not something more serious that needs extra attention.
Good luck - I hope he is much better soon.