A.H.
If you think they are seasonal allergies (age 3 is about when they start to show up) you could try Claritin or a children's decongestant - I believe Pediacare makes one that is just the decongestant and doesn't have a bunch of other medicines in it. However it could also be an allergy to something in the house too - if it is more common when he is sleeping, does he have a down pillow? Has there been a new pet introduced recently? These are other factors that could be involved. Also, if you keep him inside for a day, does he seem less congested than if you have him outside all day? These are ways you can try to start narrowing down if it is allergies and, if so, which ones they might be.