AC and window open

I didn’t have a clue it would cost us that much more when both of us decided to let my father-in-law stay with us on a everyday basis. A little extra food in addition to a little more on the water bill was to be expected but he is costing us a ton of money on our electric bill every month. He doesn’t have access the control unit, so it’s not that. However he typically opens the window in his room in addition to leaves it open. Night and day, each of us have air conditioner literally pumping out the window. I’m pretty much always telling him to close the window in addition to he does but then he opens it right back up a couple hours later. I know that it’s open because the central air conditioner won’t switch off when the window is open. I’m at a loss as to what to do at this point. I could nail the window shut however I don’t want him to assume appreciate he is being treated like a little boy! This transition has been difficult for him too. I suppose the only thing both of us can do is get high tech vents installed in his room in addition to put an outdoor door on his room complete with weather stripping. If we split off his room from the rest of the house than all of the air conditioner won’t be going out of his room. I just don’t suppose how to do this separate from making him assume bad. Maybe I can pretend it is just like house upkeep or something like that to confuse him. I just suppose that something has to be done so both of us can all be happy in addition to live together.

ac plan