It is in your control you are the parent, you can start by not having the TV on so much, let him have a half hour when he gets in then let him read books, paly board games and go outside ..he doesnt need to run around he can play gentler games, It is you who makes the rules. If you engage him having a great game of scrabble, monpoly, card games, ask Dad to do the same be more involved.
Have tv time. Then after that time turn off the TV. Unplug it and put the control out of reach.
Some parents have resorted to actually removing the TV.
You are the parent. you set the rules.
Exercise is very important for kids with asthma. Use ventolin, if necessary...