Such a terrible thing to have happened to your first and only child. I am surprised that you chose to travel with such a young baby, regardless of whether airlines allow it or not.
You said in the newspaper article, you "could feel a drop in oxygen level in the aircraft". I don't believe that you can feel the "oxygen level change", but you can definitely feel the air pressure change.
The pressure change is a real and constant threat on any airline to the health of anyone, let alone a young baby. Before my last flight, I felt like I had water in my ears, no pain or discomfort - had gone alternated between the swimming pool and sauna for 3 hours. On takeoff, it started hurting tremendously and hurt the entire trip. Upon landing, I went to the hospital at the Seoul international airport (almost all international airports have hospitals), whereupon the doctor showed me pictures of my ear (discolored and swollen) and told me that it may have been sensitive or already slightly damaged, but the pressure change in the takeoff caused "catastrauphic damage" to an already damaged eardrum. He did not want me to fly for a few days, but I had to. I would think that a doctor, or parents of a 4 month old baby, would think more than twice about travelling with the infant.
I would hate to think that the doctor who said the baby could fly did not take the susceptibility of babies to such pressure changes.