First, find out what he's allergic to.
Second, if he has a lot of trouble sleeping he may have a common dust mite allergy. To help with that, start with
mattress casings which keep the dust mites in the bedding and away from your son. Dust mites live off of dead skin cells and some humans are allergic to their excrement. GROSS! You will also need pillow casings.
I wash all my bedding in
De-Mite which kills all the dust mites. I also have a detergent that kills all allergins.
Aller-safe allergen spray on curtains and other cloth like the couch can help as well.
Get a good air cleaner with a HEPA filter and put it in his bedroom. A humidifier or an air freshener will only annoy his allergies. Keep his bedroom door closed.
The best solution is just take him to an allergist and find out what he's allergic to. The allergist will help you find the right solutions.