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The condition of allergy is curable. Sometimes it is completely cured with time sometimes partially depends on some factor and baby's overall growth.
As your baby grows up, his immunity will become stronger, and the lungs will grow accordingly, and the conditions improve. Once the baby reaches the age of six or seven and will able to understand your instructions, then you can start him breathing exercises. You can also consult a yoga teacher or physical trainer for help or suggestion regarding breathing exercises.
Various breathing exercises will make his lungs stronger and will help in preventing infection. Anulom vilom pranayama is one of them, and also a stretching spring comes in market, this spring has two or three strings. Initially, you will start with one string and ask him to stretch the string as much as he can with deep inspiration or inhalation then tell him to exhale slowly and also get back the spring to normal, repeat this exercise for some time daily. But all these measures are very far from now.
Right now you can only adopt preventive measures while the change of weather or whenever there is some unusual weather or somebody around him with a respiratory infection.
In preventive measures keeps his body warm during the change of season, whenever you feel a mild cough start giving him steam, start giving him concoction before the change of weather and give him healthy and fresh food. When he is fine, then give him citrus fruits and other fruits (orange, amla or Indian gooseberry, etc.,)as they boost immunity and all other foods which are good for health. Always keep your hand clean before making his food or before feeding him.
Every month give him half teaspoon of honey once a day for 10 to 15 days and then stop it after that. After a gap of 10 or 15 days again repeat it. It will help him boosting immunity and also control his allergy. But make sure the honey should be pure.
Promote him for more outdoor activities as physical exercise will also improve his health. All these things will help him a lot.