F.R.
I know that Kim P is trying to help, but please don't give your baby any medication, especially benadryl, without talking to your doctor first.
Some things that can help are propping the baby up when he's sleeping. They sell angled sleepers for infants to keep the head above the body. Or you can just pile some little blankets under him to have an angled form. Laying them flat lets things run right into their little sinuses. Also a cool mist humidifier can help. Keep it on in his room. I only turned mine off to clean it and to refill the water. The other suggestions of saline is great. He will outgrow this. And one more suggestion is the vapor plugs. Sudacare makes them and they plug right into the wall to emit the vapors. They make baby vapor rub if you can't find the plug. And the allergy suggestion could be true. Try to keep any fabrics clean and free of pet hair/dust/smoke. If you have ceiling fans, they may need to be wiped down.
Good luck with your little guy!