When do babies breathing start to stabilize,My baby is 3mos&6days,I can still see his chest rise and fall fast?

His chest rise and fall fast, you can physically see it. At first I was thinking maybe something is wrong. He feels good, laugh, cries, eats , poop, pee and play with me like other babies do. He w2as born 37weeks but his doctor says he develops fast just like 40 weeks old babies. "You know what they say babies not up to 40 weeks always have respiration issues" I always think about it and pray it is all logic. Can a mother who know much about babies tell me if his breathing is normal cause I have booked an appointment to ask his doctor.