I notice that you're in Australia, i.e. summer and presumably pretty hot at that. How hot is it where you are?
How long does this problem go on for, and when exactly does it occur? For instance, if I have an afternoon nap, I am often cold from exhaustion when I start the nap, so I pile on the bedclothes, but then I warm up during the nap and can wake up overheated. I don't recall this problem ever happening to me at night, when the body temperature naturally drops.
When he's lying down, is he under blankets or bedclothes, and how much is he wearing? I find that my feet are crucial for this sort of thing, so that if I'm too cold I put on extra socks and put a heat pad by my feet, and if I'm too hot I have my bare feet sticking out from under the covers. I also find that cotton can work better for bedcoverings when regulating my body temperature, e.g. a cotton quilt (handmade sort of quilt, I'm not talking about a duvet/doona/eiderdown here), and that my temperature is less likely to charge all over the place if I sleep naked. I also notice that my hormones can be a factor if my temperature is all over the place, but unless he's a transman and you didn't mention this, female sex hormonal fluctuation won't be a factor here.
This might be easier if he were to post himself, you know. Is he an adult, or at least a child old enough to make a forum post?