Wheezing and congestion is actually not a sign of allergies in dogs, but rather of some type of respiratory illness. Allergies in dogs cause itchiness, as opposed to the type of hay fever that we typically see in people with seasonal allergies.
If your dog is showing these respiratory signs, then I would not recommend using any antihistamines on him. I would take him to the vet ASAP for an examination to make sure that he is not having some type of infectious bronchitis. If this is ruled out, then the doctor can determine if your dog may have a condition called COPD (chronic obstructive pulmonary disease). This is not treated with antihistamines, but rather with bronchodilators and sometimes steroids.
I am attaching a client information handout that I use in my practice that discusses COPD in greater detail. I hope that you find it useful.Click Here
I hope that this information is of help to you, and I iwsh you the best of luck with your dog. Please let me know if I can be of further assistance.
P.S.--Zyrtec is not widely used in veterinary medicine, and because of the limited clinical experience with it, there is very little information on how well tolerated it is in dogs. There are some dosage recommendations for it for treating atopic dermatitis in dogs, but not for treating rhinosinusitis. For this reason, I would not recommending using this on your dog.