What Could Possibly Have Caused My 7 Year Olds Son

What could possibly have caused my 7 year olds son drastic weight gain in such a short time span? in about a 2 1/2 month span my son went from 57lbs to 97lbs. I took him to the doctor but he did not seem to be of much concern for it concern. I am however because during this 3 months he was having breathing issues and was diagnosed with asthma because of it. The testing to find out if he had exercise induced asthma though
this was so concerning for me. the process of testing for asthma was just not done very well at all. Also he was not breathing well I believe because of whatever was causing the drastic weight gain. Now though his breathing issues seem fine and he doesn't use his inhaler even at soccer practice for an hour. However he seems to struggle and has to sit down randomly which he says is because his leg hurts. He will be 8 in May. He has always been a very active child. He does not sit around on the couch and eat junk food. So should I take him elsewhere for further testing?