Thank you for your query.
The thigh pain is not related to the asthma
or the drugs that the boy is taking. There should be some other cause of the pain which can be found by history taking and examination by the treating doctor. I suggest that you consult your doctor who will be in a better position to guide you.
The medicines that the boy is taking are safe. However, every medicine has a side effect and they should be taken under medical supervision. The doses have to be adjusted for the growing boy and as mentioned earlier do consult your doctor at regular intervals so that the doses can be adjusted.
I hope I have answered your query to your satisfaction.
Wishing the boy all the best.