Brief Answer:
Yes; you can add cough syrup formula...
Detailed Answer:
Hi,
I understand your concerns. Symptoms of allergies take a while to respond especially when symptoms involve upper respiratory tract. If cough is troubling him a lot you can add cough syrup formula to the current regimen. As he tolerated piriton well earlier, you can safely add this to the existing treatment.
In addition you may use saline nasal sprays to relieve blocked nose. Encourage him to drink plenty of warm fluids. Watch the progress of symptoms and consider visiting the doctor as and when needed.
Hope this helps. Let me know if you need clarifications.
Regards