Let me ask you a few questions:
1. How is his appetite?
2. How is his energy level?
3. What breed is he?
4. What antihistamine is he taking? What mg?
5. How much does he weigh?
Thank you for the information.
A few more questions:
How much does he weigh?
Has he had a recent heartworm test?
Is he on heartworm prevention?
If it were an allergic cough the medication would be working by now. Heartworms also cause a cough, so I just wanted to be sure he was covered there - sounds like he is. The other causes of cough in a young dog is an infection or certain parasites. I would definitely call your vet and let them know things haven't improved. They may want to place him on an antibiotic. There are other infections that can cause a cough besides kennel cough, so it may be something else.
Please let me know if you have more questions,