I’ll definitely repeat suggestions already shared by others, but hey, they’re good ones.
Keeping him comfortable is the right approach—older people really do feel the cold more, especially if they’re sitting still for long periods, so it’s not a weakness, just physiology. There are plenty of ways to help without running the heating all day.
If he’s in one spot, it’s cheaper to warm the person, not the whole room. An oil-filled radiator near his chair gives steady heat and is safer for long use than fan heaters, while halogen heaters are best avoided. Direct warmth works even better: electric throws or heated blankets cost very little to run, hot water bottles are simple but effective, and thermal socks, slippers, and layers keep heat in. Chair placement matters too—if it’s near a window, door, or outside wall, he’ll feel colder. A draft excluder, a blanket over the back of the chair, or moving it slightly away from a cold wall can make a big difference. Warmth from the inside helps as well—regular hot drinks and warm meals go a long way. Keeping the house at 21° is already great; if he’s still cold, it’s not about spending more, just giving targeted warmth.
If the chill persists, mentioning it to the GP could be wise, as issues like poor circulation or anaemia can make someone feel colder. In these tough cost-of-living times, local heat and layers remain the kindest and most affordable solution.