Fast Breathing or Shortness of Breath.

My son has had intermittent fast Breathing or Shortness of breath issues for the past 1 month. Sometimes it happens 4-5 times in 1 hour and sometimes it doesn't occur for many hours. There is no issue while he sleeps. His X-Ray report, CECT Report and Laryngoscopy is normal. The only thing we did was milk change and we thought it was a milk allergy. We have stopped the milk and started giving Similac isomil there is no issue for 5 days but he has cold and chest congestion during these days along with fever. Now he has a little bit of mucus in his nose and these intermittent fast Breathing happening again. Not sure what exactly is happening with him. Is it Allergy, Chest Issue, Anxiety or anything else. No idea what type of doctor I reach out too.