20 month old with stuffy nose?

my 20 month old son has a stuffy nose and its also runny. He is so miserable right now it started yesterday cuz my dumb brother sneezed on him on easter. at night is the worst when he is trying to go to bed he has to breathe thru his mouth but he sucks on a pacifier which makes it more difficult cuz he cant suck on it and breath out his mouth. will it be okay if i give him some benadryl to him sleep at night?? if so how much should i give him. I have a humidifier but that doesn't seem to work with him. What else should i do