I am really concerned that your child is suffering from asthma
For addiction to oocur, drugs should reach brain, it should give a pleasurable sensation and a craving for the drug. Asthma medications like montair donot cause addiction. So don't be afraid that your child will be addicted. It doesn't have any long term side effects and they don't affect immunity.
If you are so much worried about taking oral medications and their side effects you can consult your physician about taking inhaler medications which reach only lungs without much side effetcs.
Good news is a good number of children by age of 6 years may outgrow asthma. So don't worry he will be alright. Just give him preventor medications, preferably inhalers as they are with minimal side effects.
Dr C Balaji