Coughing/congestion at 5 months

My 5 month old has been super congested since birth. We just started on the liquid off brand of Zyrtec for his allergies (dad has super bad allergies) well he will have like these coughing spasms at night where he literally will cough for 30 min straight. Took him to his pediatrician and urgent care and they all say it’s just congestion due to his lungs and breathing sound good! I’m just super worried. I hate seeing him like this. We are elevating his bassinet mattress so he’s not laughing flat. We use a humidifier. Not sure what else I can do. Any suggestions?? Would be greatly appreciated.