when the room is warm, more of his blood is near the surface of his skin so water can evaporate out and cause cooling of the skin (and blood). When the room is cool, the blood is kept more in the muscles and organs, so he has enough blood in his brain to remain conscious.
Older people often get what is called orthostatic hypotension, which just means that if they stand suddenly from sitting or lying down, they are likely to black out or faint. (We young kids can do the same, it just doesn’t happen as often.)