Is there a rail on the floor that the door slides on? If so, you could make a heavy duty stick just long enough to let the door open a little to let some air in for you guys, but not open far enough for Boomer to get out. Get the right length stick to lay in the rail. When the door opens so far it will hit the stick and Boomer won't be able to open it any further - unless he's Super Boomer!

If you don't understand what I'm trying to say, ask because I think this could be the solution to your problem. It's kind of like what hotels use for security for sliding doors.....unfortunately I don't have a picture....