You can use Children's Benadryl to decongest him. I was told my son (2 years, 32lbs) can have 1 tablespoon, but I only give him 1/2 tablespoon because it knocks him out. My pediatrician recommended this for bedtime because kids can not have cold meds.
I do recommend checking with your doctor to see if this is safe if your son has never taken Benadryl, I do not advise giving new drugs without the doctor's OK.
I don't know why his nose is running, he could have a cold without having a fever, he is due for his molars, it could be anything really.
Good luck
EDIT: I came back to add this about allergies and saw the next poster mentioned them. My son has springtime allergies and he was put on Loratadine syrup (generic Claritin)until the end of May. Runny nose was his only symptom for weeks and we kept treating it like a cold until the doctor finally said if it isn't getting better it must be allergies. I thought he should be tested, but she said if the Loratadine works we wouldn't have to put him through testing until he was older and this year was really really bad for allergies and he may never have symptoms again.