May I assume his room is separate from yours?
I figure the reason for his aggressiveness when he goes downstairs is he is moving into a place that is not his pre-established territory. Since he is innately a prey species, his guard goes way up. BTW all igs are obligate loners until a person trains them into trusting people. According to anapsid.org, igs of any age can be trained/tamed.
In the wild, igs actively seek out cool spaces.
It's too bad he can't freely go downstairs.
The first thing you can do, if the others in your home are OK with it, is to train him to be comfortable while he is downstairs. To do that you merely have to repeatedly bring him down so he can get more and more familiar with the sights, sounds, and odors. Place treats on the floor in several places. If there is a high shelf for him with some kind of ramp he will become more relaxed faster. It either needs an on ramp and an off ramp, or the shelf needs to be a minimum of 18 inches deep so he can turn around without breaking his tail.
You mention that your room has AC. Is it possible to put a window AC in iggy's room?
Are you handy enough to make a DIY air cooler for him? check this out:
Online there are very simple ones too.
I hope that something here helps.