Heating in a shared office

I'm at the end of my rope with this, which may sound petty to some but it's really affecting me. I share an office which is quite small (c. 5mx5m). It contains two humans, two PCs and a printer. My colleague turns up the heating thermostat when I am not here to witness him doing so. I then turn it down again and I sometimes open the window because I think I'll pass out otherwise. I tried to discuss this issue with him some months ago but he would not even look at me, let alone "engage". He kept his eyes down and mumbled something about keeping it "in the middle". He is a grown man in his late 40s who does not know how to dress appropriately for winter and spring weather. Instead he wants to wear a cotton shirt and have the heat set to maximum (yes, maximum). I find this intolerable. For a few weeks I changed office but that option is no longer possible. I came back after lunch today to find the thermostat turned up again. My face is beet red. I've opened the window now which I know bugs him. How do we resolve this?