I understand your son has allergic rhino-bronchitis
. Nasal sprays is the mainstay in the treatment of allergic rhinitis
. Since fenistil is a antihistamine it can be used in the treatment of allergic rhinitis.
These antihistaminic drugs are known to cause some sedation. Thats why your son is tired. Try reducing the dosage of Fenistil to the half of what has been adviced. It may help.
if it doesn't help consult your doctor for changing the medications