I'd try a saline solution at night and maybe periodically throughout the day.
And I also agree with washing his pillow and bed spread.
In addition, I'd consider getting allergen mattress and pillow case covers in case he is allergic to dust mites.
If you want to try allergy medicine and Claritin isn't working, you could try Zyrtec or see an pediatric allergist and see what he/she suggests. Some medicines work better than others for each person. It seems to be that way with Claritin and Zyrtec. Some swear by one and others swear by the other.
One of my sons seems to have seasonal allergies and he has been doing better now that we've given him Zyrtec. I have tons of of outdoor allergies. I do a nasal inhaler and Zyrtec. If you talk to a pediatric allergist and your son ends up on a nasal inhaler, keep in mind that different inhalers also work better than others depending on the person. If one nasal inhaler doesn't work, ask the allergist for another brand. The allergist should be able to look up your son's nose and see if an inhaler would help as they can tell if the nasal passages are red and swollen.